Access ChromotingHost::clients_ only on network thread.

Previously ChromotingHost was doing some work on the main thread and
some on the network thread. |clients_| and some other members were
accessed without lock on both of these threads. Moved most of the
ChromotingHost activity to the network thread to avoid possible
race conditions.

BUG=96325
TEST=Chromoting works

Review URL: http://codereview.chromium.org/8495024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109556 0039d316-1c4b-4281-b951-d872f2087c98
 void ScreenRecorder::AddConnection(
     scoped_refptr<ConnectionToClient> connection) {
  DCHECK(network_loop_->BelongsToCurrentThread());
  connections_.push_back(connection);

   capture_loop_->PostTask(
       FROM_HERE, base::Bind(&ScreenRecorder::DoInvalidateFullScreen, this));
 }

 void ScreenRecorder::AddConnection(
     scoped_refptr<ConnectionToClient> connection) {
   capture_loop_->PostTask(
       FROM_HERE, base::Bind(&ScreenRecorder::DoInvalidateFullScreen, this));
  network_loop_->PostTask(
      FROM_HERE, base::Bind(&ScreenRecorder::DoAddConnection,
                            this, connection));
 }
