Add MessagePipe interface. Use it in ChannelDispatcherBase.

remoting/protocol/client_video_dispatcher.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "remoting/protocol/client_video_dispatcher.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/callback_helpers.h"
@@ skipping 48 matching lines @@
     PendingFramesList::iterator pending_frame) {
   // Mark the frame as done.
   DCHECK(!pending_frame->done);
   pending_frame->done = true;
 
   // Send VideoAck for all packets in the head of the queue that have finished
   // rendering.
   while (!pending_frames_.empty() && pending_frames_.front().done) {
     VideoAck ack_message;
     ack_message.set_frame_id(pending_frames_.front().frame_id);
-    writer()->Write(SerializeAndFrameMessage(ack_message), base::Closure());
+    message_pipe()->Send(&ack_message, base::Closure());
     pending_frames_.pop_front();
   }
 }
 
 }  // namespace protocol
 }  // namespace remoting