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 dea6cf5180b050157f46270fba888fa0954a48f5..67dfc0ad3b910a5432efece870201eeab2e4823e 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -57,6 +57,7 @@
 #include "content/renderer/gpu/gpu_benchmarking_extension.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"
@@ -245,6 +246,8 @@ void RenderThreadImpl::Init() {
 
   AddFilter(new IndexedDBMessageFilter);
 
+  audio_renderer_mixer_manager_.reset(new AudioRendererMixerManager());
+
   content::GetContentClient()->renderer()->RenderThreadStarted();
 
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();