Turn on audio mixer by default.

content/browser/renderer_host/media/audio_renderer_host_unittest.cc

 // 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.
 
 #include "base/bind.h"
 #include "base/environment.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop.h"
 #include "base/process_util.h"
 #include "base/sync_socket.h"
@@ skipping 341 matching lines @@
 }
 
 TEST_F(AudioRendererHostTest, SetVolume) {
   if (!IsRunningHeadless())
     EnableRealDevice();
 
   Create();
   SetVolume(0.5);
   Play();
   Pause();
-
-  // Expect the volume is set.
-  if (IsRunningHeadless()) {
-    EXPECT_EQ(0.5,
-              media::FakeAudioOutputStream::GetCurrentFakeStream()->volume());
-  }
   Close();
 }
 
 // Simulate the case where a stream is not properly closed.
 TEST_F(AudioRendererHostTest, CreatePlayAndShutdown) {
   if (!IsRunningHeadless())
     EnableRealDevice();
 
   Create();
   Play();
@@ skipping 26 matching lines @@
     EnableRealDevice();
 
   Create();
   Play();
   SimulateError();
   Close();
 }
 
 
 // TODO(hclam): Add tests for data conversation in low latency mode.