Reland "Leverage dispatch_queue_create_with_target when possible."

This is a reland of de86381161651816c078adeb354902b15d03a35b

Original change's description:
> Leverage dispatch_queue_create_with_target when possible.
> 
> Replacing dispatch_queue_create followed by
> dispatch_set_target_queue with dispatch_queue_create_with_target
> is claimed to be source of GCD performance improvement:
> https://developer.apple.com/videos/play/wwdc2017/706/
> Video since 40 min. Slides since 199.
> 
> Bug: webrtc:9055
> Change-Id: I0136f7faaef0951a7ad243bc8772f3ee952d5470
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168491
> Reviewed-by: Tommi <tommi@webrtc.org>
> Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
> Commit-Queue: Yura Yaroshevich <yura.yaroshevich@gmail.com>
> Cr-Commit-Position: refs/heads/master@{#30781}

Bug: webrtc:9055
Change-Id: I36b0b6423c81c0497f66f7c993741c33ff6ec5ba
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/170443
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30821}

5 files changed, 65 insertions, 4 deletions

diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 5cb3fea8da..2e4138e458 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -487,6 +487,7 @@ if (is_mac || is_ios) {
       ":checks",
       ":logging",
       "../api/task_queue",
+      "system:gcd_helpers",
       "//third_party/abseil-cpp/absl/strings",
     ]
   }
diff --git a/rtc_base/system/BUILD.gn b/rtc_base/system/BUILD.gn
index 937fec11e2..79cb301038 100644
--- a/rtc_base/system/BUILD.gn
+++ b/rtc_base/system/BUILD.gn
@@ -60,6 +60,14 @@ if (is_mac || is_ios) {
     deps = [ "..:checks" ]
     libs = [ "Foundation.framework" ]
   }
+
+  rtc_library("gcd_helpers") {
+    sources = [
+      "gcd_helpers.h",
+      "gcd_helpers.m",
+    ]
+    include_dirs = [ "../.." ]
+  }
 }
 
 rtc_source_set("thread_registry") {
diff --git a/rtc_base/system/gcd_helpers.h b/rtc_base/system/gcd_helpers.h
new file mode 100644
index 0000000000..a8df0a9d83
--- /dev/null
+++ b/rtc_base/system/gcd_helpers.h
@@ -0,0 +1,29 @@
+/*
+ *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_SYSTEM_GCD_HELPERS_H_
+#define RTC_BASE_SYSTEM_GCD_HELPERS_H_
+
+#include <dispatch/dispatch.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+DISPATCH_RETURNS_RETAINED DISPATCH_WARN_RESULT DISPATCH_NOTHROW dispatch_queue_t
+RTCDispatchQueueCreateWithTarget(const char* label,
+                                 dispatch_queue_attr_t attr,
+                                 dispatch_queue_t target);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RTC_BASE_SYSTEM_GCD_HELPERS_H_
diff --git a/rtc_base/system/gcd_helpers.m b/rtc_base/system/gcd_helpers.m
new file mode 100644
index 0000000000..ff113266a1
--- /dev/null
+++ b/rtc_base/system/gcd_helpers.m
@@ -0,0 +1,22 @@
+/*
+ *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "rtc_base/system/gcd_helpers.h"
+
+dispatch_queue_t RTCDispatchQueueCreateWithTarget(const char *label,
+                                                  dispatch_queue_attr_t attr,
+                                                  dispatch_queue_t target) {
+  if (@available(iOS 10, macOS 10.12, tvOS 10, watchOS 3, *)) {
+    return dispatch_queue_create_with_target(label, attr, target);
+  }
+  dispatch_queue_t queue = dispatch_queue_create(label, attr);
+  dispatch_set_target_queue(queue, target);
+  return queue;
+}
\ No newline at end of file
diff --git a/rtc_base/task_queue_gcd.cc b/rtc_base/task_queue_gcd.cc
index cb516cc4cc..2276f635c5 100644
--- a/rtc_base/task_queue_gcd.cc
+++ b/rtc_base/task_queue_gcd.cc
@@ -24,6 +24,7 @@
 #include "api/task_queue/task_queue_base.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/system/gcd_helpers.h"
 
 namespace webrtc {
 namespace {
@@ -67,16 +68,16 @@ class TaskQueueGcd : public TaskQueueBase {
 };
 
 TaskQueueGcd::TaskQueueGcd(absl::string_view queue_name, int gcd_priority)
-    : queue_(dispatch_queue_create(std::string(queue_name).c_str(),
-                                   DISPATCH_QUEUE_SERIAL)),
+    : queue_(RTCDispatchQueueCreateWithTarget(
+          std::string(queue_name).c_str(),
+          DISPATCH_QUEUE_SERIAL,
+          dispatch_get_global_queue(gcd_priority, 0))),
       is_active_(true) {
   RTC_CHECK(queue_);
   dispatch_set_context(queue_, this);
   // Assign a finalizer that will delete the queue when the last reference
   // is released. This may run after the TaskQueue::Delete.
   dispatch_set_finalizer_f(queue_, &DeleteQueue);
-
-  dispatch_set_target_queue(queue_, dispatch_get_global_queue(gcd_priority, 0));
 }
 
 TaskQueueGcd::~TaskQueueGcd() = default;