[chromecast] Moves CastAudioOutputStream::Backend to CMA thread.

chromecast/browser/cast_content_browser_client.cc

Index: chromecast/browser/cast_content_browser_client.cc
diff --git a/chromecast/browser/cast_content_browser_client.cc b/chromecast/browser/cast_content_browser_client.cc
index 3d7f35326187293adc047c8518f0f42586f7ecfc..b0d3c823040806037649ca46a786a14e2a0c4fe0 100644
--- a/chromecast/browser/cast_content_browser_client.cc
+++ b/chromecast/browser/cast_content_browser_client.cc
@@ -37,7 +37,7 @@
 #include "chromecast/browser/url_request_context_factory.h"
 #include "chromecast/common/global_descriptors.h"
 #include "chromecast/media/audio/cast_audio_manager.h"
-#include "chromecast/media/cma/backend/media_pipeline_backend_factory.h"
+#include "chromecast/media/cma/backend/media_pipeline_backend_factory_impl.h"
 #include "chromecast/media/cma/backend/media_pipeline_backend_manager.h"
 #include "chromecast/public/media/media_pipeline_backend.h"
 #include "components/crash/content/app/breakpad_linux.h"
@@ -177,7 +177,7 @@ CastContentBrowserClient::GetMediaPipelineBackendFactory() {
   DCHECK(GetMediaTaskRunner()->BelongsToCurrentThread());
   if (!media_pipeline_backend_factory_) {
     media_pipeline_backend_factory_.reset(
-        new media::MediaPipelineBackendFactory(
+        new media::MediaPipelineBackendFactoryImpl(
             media_pipeline_backend_manager()));
   }
   return media_pipeline_backend_factory_.get();
@@ -197,9 +197,15 @@ CastContentBrowserClient::media_pipeline_backend_manager() {
 std::unique_ptr<::media::AudioManager>
 CastContentBrowserClient::CreateAudioManager(
     ::media::AudioLogFactory* audio_log_factory) {
+  // TODO(alokp): Consider switching off the mixer on audio platforms

[slan, 2017/05/30 16:26:01]

Is this comment useful? Aren't we blocked on dmix issues?

[alokp, 2017/05/30 23:19:33]

Not really - We can switch off the CastAudioManager mixer today on audio
platforms because mixer is implemented in CMA-Alsa wrapper, which sits upstream
of alsa/dmix.

[slan, 2017/05/30 23:44:39]

Ah, I see what you mean - disable the mixer whenever the alsa CMA backend is
used. Vendors (who may or may not use the alsa CMA backend) should still get
this functionality. 

+  // because we already have a mixer in the audio pipeline downstream of
+  // CastAudioManager.
+  bool use_mixer = true;
   return base::MakeUnique<media::CastAudioManager>(
       base::MakeUnique<::media::AudioThreadImpl>(), audio_log_factory,
-      media_pipeline_backend_manager());
+      base::MakeUnique<media::MediaPipelineBackendFactoryImpl>(
+          media_pipeline_backend_manager()),
+      GetMediaTaskRunner(), use_mixer);
 }
 
 std::unique_ptr<::media::CdmFactory>