Put browser side mixing back behind a flag.

BUG=140247
TEST=Tried --enable-audio-mixer flag and observed regression.

Review URL: https://chromiumcodereview.appspot.com/10834159

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@149906 0039d316-1c4b-4281-b951-d872f2087c98
void AudioManagerBase::Shutdown() {
  scoped_ptr<base::Thread> audio_thread;
  {
    base::AutoLock lock(audio_thread_lock_);
    audio_thread_.swap(audio_thread);
  }

  if (!audio_thread.get())
    return;

  CHECK_NE(MessageLoop::current(), audio_thread->message_loop());

  audio_thread->message_loop()->PostTask(FROM_HERE, base::Bind(
      &AudioManagerBase::ShutdownOnAudioThread,
      base::Unretained(this)));

  audio_thread->Stop();
}

void AudioManagerBase::Shutdown() {
  scoped_ptr<base::Thread> audio_thread;
  {
    base::AutoLock lock(audio_thread_lock_);
    audio_thread_.swap(audio_thread);
  }

  if (!audio_thread.get())
    return;

  CHECK_NE(MessageLoop::current(), audio_thread->message_loop());

  audio_thread->message_loop()->PostTask(FROM_HERE, base::Bind(
      &AudioManagerBase::ShutdownOnAudioThread,
      base::Unretained(this)));

  audio_thread->Stop();
}
