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
WebKit::WebPeerConnectionHandler* MediaStreamImpl::CreatePeerConnectionHandler(
    WebKit::WebPeerConnectionHandlerClient* client) {
  DCHECK(CalledOnValidThread());
  if (peer_connection_handler_) {
    DVLOG(1) << "A PeerConnection already exists";
    return NULL;
  }
  EnsurePeerConnectionFactory();

  peer_connection_handler_ = new PeerConnectionHandler(
      client,
      this,
      dependency_factory_.get());

  return peer_connection_handler_;
}

WebKit::WebPeerConnectionHandler* MediaStreamImpl::CreatePeerConnectionHandler(
    WebKit::WebPeerConnectionHandlerClient* client) {
  DCHECK(CalledOnValidThread());
  if (peer_connection_handler_) {
    DVLOG(1) << "A PeerConnection already exists";
    return NULL;
  }
  EnsurePeerConnectionFactory();

  peer_connection_handler_ = new PeerConnectionHandler(
      client,
      this,
      dependency_factory_.get());

  return peer_connection_handler_;
}
