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::Init() {
  base::AutoLock lock(audio_thread_lock_);
  DCHECK(!audio_thread_.get());
  audio_thread_.reset(new base::Thread("AudioThread"));
  CHECK(audio_thread_->Start());
}

void AudioManagerBase::Init() {
  base::AutoLock lock(audio_thread_lock_);
  DCHECK(!audio_thread_.get());
  audio_thread_.reset(new base::Thread("AudioThread"));
  CHECK(audio_thread_->Start());
}
