Use the lock when accessing the buffer object.

BUG=69195
TEST=play Z-Type for hours :)

Review URL: http://codereview.chromium.org/6157007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@71211 0039d316-1c4b-4281-b951-d872f2087c98
void AudioOutputController::DoSetVolume(double volume) {
  DCHECK_EQ(message_loop_, MessageLoop::current());

  volume_ = volume;

  if (state_ != kPlaying && state_ != kPaused && state_ != kCreated)
    return;

  stream_->SetVolume(volume_);
}

void AudioOutputController::DoSetVolume(double volume) {
  DCHECK_EQ(message_loop_, MessageLoop::current());

  volume_ = volume;

  if (state_ != kPlaying && state_ != kPaused && state_ != kCreated)
    return;

  stream_->SetVolume(volume_);
}
