Add OSX aggregate audio device support for best performance.

media/audio/mac/audio_manager_mac.h

Index: media/audio/mac/audio_manager_mac.h
===================================================================
--- media/audio/mac/audio_manager_mac.h	(revision 191870)
+++ media/audio/mac/audio_manager_mac.h	(working copy)
@@ -12,6 +12,7 @@
 #include "base/compiler_specific.h"
 #include "base/message_loop_proxy.h"
 #include "media/audio/audio_manager_base.h"
+#include "media/audio/mac/aggregate_device_manager.h"
 #include "media/audio/mac/audio_device_listener_mac.h"
 
 namespace media {
@@ -75,6 +76,8 @@
   int current_sample_rate_;
   AudioDeviceID current_output_device_;
 
+  AggregateDeviceManager aggregate_device_manager_;
+
   DISALLOW_COPY_AND_ASSIGN(AudioManagerMac);
 };