Enable renderer side mixing behind a flag.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 99988bee028a316b032ea95bcdec8200d550661e..0cb91368ccc5c37f0bc30a9faf469f5268b5da20 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -55,8 +55,10 @@
 #include "content/renderer/dom_storage/webstoragenamespace_impl.h"
 #include "content/renderer/gpu/compositor_thread.h"
 #include "content/renderer/gpu/gpu_benchmarking_extension.h"
+#include "content/renderer/media/audio_hardware.h"
 #include "content/renderer/media/audio_input_message_filter.h"
 #include "content/renderer/media/audio_message_filter.h"
+#include "content/renderer/media/audio_renderer_mixer_manager.h"
 #include "content/renderer/media/media_stream_center.h"
 #include "content/renderer/media/video_capture_impl_manager.h"
 #include "content/renderer/media/video_capture_message_filter.h"
@@ -111,6 +113,7 @@ using WebKit::WebScriptController;
 using WebKit::WebSecurityPolicy;
 using WebKit::WebString;
 using WebKit::WebView;
+using content::AudioRendererMixerManager;
 using content::RenderProcessObserver;
 
 namespace {
@@ -750,6 +753,17 @@ RenderThreadImpl::GetGpuVDAContext3D() {
   return gpu_vda_context3d_->AsWeakPtr();
 }
 
+content::AudioRendererMixerManager*
+RenderThreadImpl::GetAudioRendererMixerManager() {
+  if (!audio_renderer_mixer_manager_.get()) {
+    audio_renderer_mixer_manager_.reset(new AudioRendererMixerManager(
+        audio_hardware::GetOutputSampleRate(),
+        audio_hardware::GetOutputBufferSize()));
+  }
+
+  return audio_renderer_mixer_manager_.get();
+}
+
 #if defined(OS_WIN)
 void RenderThreadImpl::PreCacheFont(const LOGFONT& log_font) {
   Send(new ChildProcessHostMsg_PreCacheFont(log_font));