Explicitly stopping thread in MediaStreamImpl dtor to avoid any racing issues.

This may solve the below bugs.

BUG=112408,111202
TEST=content_unittests

Review URL: https://chromiumcodereview.appspot.com/9307058

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@120222 0039d316-1c4b-4281-b951-d872f2087c98
void MediaStreamImpl::ClosePeerConnection() {
  DCHECK(CalledOnValidThread());
  video_renderer_ = NULL;
  peer_connection_handler_ = NULL;
  MediaStreamTrackPtrMap::const_iterator it = local_tracks_.begin();
  for (; it != local_tracks_.end(); ++it)
    it->second->set_state(webrtc::MediaStreamTrackInterface::kEnded);
}

void MediaStreamImpl::ClosePeerConnection() {
  DCHECK(CalledOnValidThread());
  video_renderer_ = NULL;
  peer_connection_handler_ = NULL;
  MediaStreamTrackPtrMap::const_iterator it = local_tracks_.begin();
  for (; it != local_tracks_.end(); ++it)
    it->second->set_state(webrtc::MediaStreamTrackInterface::kEnded);
}
