Revert "Removes lock release in PacedSender callback."

This reverts commit 6b9c60b06d04bc519195fca1f621b10accfeb46b.

Reason for revert: Breaks downstream test

Original change's description:
> Removes lock release in PacedSender callback.
> 
> The PacedSender currently has logic to temporarily release its internal
> lock while sending or asking for padding.
> This creates some tricky situations in the pacing controller where we
> need to consider if some thread can enter while we the process thread is
> actually processing, just temporarily busy sending.
> 
> Since the pacing call stack is no longer cyclic, we can actually remove
> this lock-release now.
> 
> Bug: webrtc:10809
> Change-Id: Ic59c605252bed1f96a03406c908a30cd1012f995
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173592
> Reviewed-by: Sebastian Jansson <srte@webrtc.org>
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31206}

TBR=sprang@webrtc.org,srte@webrtc.org

Change-Id: Ic84eee6097528d0792e3b1f90f36bc78447a0d81
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:10809
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174820
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31209}

1 files changed, 1 insertions, 0 deletions

diff --git a/test/direct_transport.cc b/test/direct_transport.cc
index f4ae047870..818480b18b 100644
--- a/test/direct_transport.cc
+++ b/test/direct_transport.cc
@@ -55,6 +55,7 @@ DirectTransport::~DirectTransport() {
 }
 
 void DirectTransport::SetReceiver(PacketReceiver* receiver) {
+  rtc::CritScope cs(&process_lock_);
   fake_network_->SetReceiver(receiver);
 }