Move MediaStreamAudioProcessor processing code to new class in media/

This moves the bulk of the implementation of MediaStreamAudioProcessor - the audio processing - to a separate class media::AudioProcessor in the media component.
MediaStreamAudioProcessor is essentially reduced to managing renderer-specific data and control flow for the audio processor:
- aec dump interface toward browser
- stats interface toward WebRTC
- playout data sink towards WebRTC audio source
... as well as enforcing renderer-specific threading expectations.

This CL widens the distance between MediaStreamAudioProcessor::WouldModifyAudio() and the logic (AudioProcessor::InitializeAudioProcessingModule()). Tests are added to enforce that the functions agree.

Notes for reviewing in Gerrit:
- audio_processor.* is branched from media_stream_audio_processor.*
  - diff vs patch set 1 to get a clean diffbase
- audio_processor_test.cc is branched from media_stream_audio_processor_test.cc
  - diff vs patch set 6 to get a clean diffbase

Tested: Made WebRTC call with a release build (+dchecks), and checked 1) audio experience 2) logs 3) echo cancellation 4) contents of diagnostic audio recordings (wav+aecdump)
Bug: chromium:1215061
Change-Id: I6c49bc71d2b1441b27199a137cb8a3204c6566b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3269221
Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Sam Zackrisson <saza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#949987}

media/base/audio_processing.h

@@ -31,7 +31,7 @@ struct MEDIA_EXPORT AudioProcessingSettings {
   bool multi_channel_capture_processing = true;
   bool stereo_mirroring = false;
 
-  // TODO(https://crbug.com/1215061): Deprecate this setting.
+  // TODO(https://crbug.com/1269723): Deprecate this setting.
   // This flag preserves the behavior of the to-be-deprecated flag / constraint
   // |AudioProcessingProperties::goog_experimental_echo_cancellation|: It has no
   // effect on what effects are enabled, but for legacy reasons, it forces APM

media/webrtc/BUILD.gn

@@ -11,6 +11,8 @@ component("webrtc") {
   sources = [
     "audio_delay_stats_reporter.cc",
     "audio_delay_stats_reporter.h",
+    "audio_processor.cc",
+    "audio_processor.h",
     "constants.h",
     "helpers.cc",
     "helpers.h",
@@ -23,6 +25,7 @@ component("webrtc") {
   deps = [
     "//base",
     "//build:chromecast_buildflags",
+    "//build:chromeos_buildflags",
     "//media",
     "//media:shared_memory_support",
     "//third_party/webrtc_overrides:webrtc_component",
@@ -41,5 +44,8 @@ source_set("unit_tests") {
     "//testing/gtest",
     "//third_party/webrtc_overrides:webrtc_component",
   ]
-  sources = [ "helpers_unittests.cc" ]
+  sources = [
+    "audio_processor_test.cc",
+    "helpers_unittests.cc",
+  ]
 }