APM: Add build flag to allow building WebRTC without APM

This CL adds a build flag to allow building the non-test parts
of WebRTC without the audio processing module.
The CL also ensures that the WebRTC code correctly handles
the case when no APM is available.

Bug: webrtc:5298
Change-Id: I5c8b5d1f7115e5cce2af4c2b5ff701fa1c54e49e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/171509
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31133}

1 files changed, 0 insertions, 1 deletions

diff --git a/audio/audio_state.cc b/audio/audio_state.cc
index 1a4fd77ed2..73366e20a8 100644
--- a/audio/audio_state.cc
+++ b/audio/audio_state.cc
@@ -41,7 +41,6 @@ AudioState::~AudioState() {
 }
 
 AudioProcessing* AudioState::audio_processing() {
-  RTC_DCHECK(config_.audio_processing);
   return config_.audio_processing.get();
 }