Enable renderer side mixing behind a flag.

content/renderer/media/render_audiosourceprovider.cc

Index: content/renderer/media/render_audiosourceprovider.cc
diff --git a/content/renderer/media/render_audiosourceprovider.cc b/content/renderer/media/render_audiosourceprovider.cc
index 039f75fb8f8c96dc3f01edf30ae50add4b65ebc3..6bd16ed576b0c4915af7b6ea15ad756885246f20 100644
--- a/content/renderer/media/render_audiosourceprovider.cc
+++ b/content/renderer/media/render_audiosourceprovider.cc
@@ -5,8 +5,13 @@
 #include "content/renderer/media/render_audiosourceprovider.h"
 
 #include "base/basictypes.h"
+#include "base/command_line.h"
 #include "base/logging.h"
 #include "content/renderer/media/audio_device_factory.h"
+#include "content/renderer/media/audio_renderer_mixer_manager.h"
+#include "content/renderer/render_thread_impl.h"
+#include "media/base/audio_renderer_mixer_input.h"
+#include "media/base/media_switches.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebAudioSourceProviderClient.h"
 
 using content::AudioDeviceFactory;
@@ -21,11 +26,24 @@ RenderAudioSourceProvider::RenderAudioSourceProvider()
       volume_(1.0),
       renderer_(NULL),
       client_(NULL) {
-  // We create the AudioDevice here using the factory. But we don't yet know
-  // the audio format (sample-rate, etc.) at this point.  Later, when
-  // Initialize() is called, we have the audio format information and call
-  // the AudioDevice::Initialize() method to fully initialize it.
-  default_sink_ = AudioDeviceFactory::Create();
+  // Only process the command line switch once.
+  static int enable_render_side_mixing = -1;

[scherkus (not reviewing), 2012/07/13 18:40:16]

don't worry about optimizing the switch lookup on subsequent object creation --
just do the HasSwitch() every time

[DaleCurtis, 2012/07/13 21:28:33]

Done.

+  if (enable_render_side_mixing < 0) {
+    const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
+    enable_render_side_mixing = static_cast<int>(cmd_line->HasSwitch(
+        switches::kEnableRendererSideMixing));
+  }
+
+  // We create an AudioRendererSink here, but we don't yet know the audio format
+  // (sample-rate, etc.) at this point.  Later, when Initialize() is called, we
+  // have the audio format information and call AudioRendererSink::Initialize()
+  // to fully initialize it.
+  if (!enable_render_side_mixing) {
+    default_sink_ = AudioDeviceFactory::Create();
+  } else {
+    default_sink_ = RenderThreadImpl::current()->
+        audio_renderer_mixer_manager()->CreateInput();
+  }
 }
 
 void RenderAudioSourceProvider::setClient(