Enable renderer side mixing behind a flag.

content/content_renderer.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'dependencies': [
     '../jingle/jingle.gyp:jingle_glue',
     '../net/net.gyp:net',
     '../ppapi/ppapi_internal.gyp:ppapi_host',
     '../ppapi/ppapi_internal.gyp:ppapi_proxy',
@@ skipping 90 matching lines @@
     'renderer/media/audio_device_thread.cc',
     'renderer/media/audio_device_thread.h',
     'renderer/media/audio_hardware.cc',
     'renderer/media/audio_hardware.h',
     'renderer/media/audio_input_device.cc',
     'renderer/media/audio_input_device.h',
     'renderer/media/audio_input_message_filter.cc',
     'renderer/media/audio_input_message_filter.h',
     'renderer/media/audio_message_filter.cc',
     'renderer/media/audio_message_filter.h',
+    'renderer/media/audio_renderer_mixer_manager.cc',
+    'renderer/media/audio_renderer_mixer_manager.h',
     'renderer/media/capture_video_decoder.cc',
     'renderer/media/capture_video_decoder.h',
     'renderer/media/media_stream_center.h',
     'renderer/media/media_stream_dependency_factory.h',
     'renderer/media/media_stream_dispatcher.h',
     'renderer/media/media_stream_dispatcher_eventhandler.h',
     'renderer/media/media_stream_impl.h',
     'renderer/media/pepper_platform_video_decoder_impl.cc',
     'renderer/media/pepper_platform_video_decoder_impl.h',
     'renderer/media/render_audiosourceprovider.cc',
@@ skipping 224 matching lines @@
     }],
   ],
   'target_conditions': [
     ['OS=="android"', {
       'sources/': [
         ['include', '^renderer/render_view_linux\\.cc$'],
       ],
     }],
   ],
 }