Turn on audio mixer by default.

media/audio/audio_manager_base.cc

Index: media/audio/audio_manager_base.cc
===================================================================
--- media/audio/audio_manager_base.cc	(revision 139782)
+++ media/audio/audio_manager_base.cc	(working copy)
@@ -5,7 +5,6 @@
 #include "media/audio/audio_manager_base.h"
 
 #include "base/bind.h"
-#include "base/command_line.h"
 #include "base/message_loop_proxy.h"
 #include "base/threading/thread.h"
 #include "media/audio/audio_output_dispatcher_impl.h"
@@ -142,12 +141,7 @@
   if (!dispatcher) {
     base::TimeDelta close_delay =
         base::TimeDelta::FromSeconds(kStreamCloseDelaySeconds);
-    const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
-    if (cmd_line->HasSwitch(switches::kEnableAudioMixer)) {
-      dispatcher = new AudioOutputMixer(this, params, close_delay);
-    } else {
-      dispatcher = new AudioOutputDispatcherImpl(this, params, close_delay);
-    }
+    dispatcher = new AudioOutputMixer(this, params, close_delay);
   }
   return new AudioOutputProxy(dispatcher);
 }