Add MessagePipe interface. Use it in ChannelDispatcherBase.

remoting/protocol/client_video_dispatcher.cc

Index: remoting/protocol/client_video_dispatcher.cc
diff --git a/remoting/protocol/client_video_dispatcher.cc b/remoting/protocol/client_video_dispatcher.cc
index f82b7bd74f77762fb7ac49af9ea7f965bd3f8e8f..7fbe9f38e24518b17cec283e589ff8b435dd03c4 100644
--- a/remoting/protocol/client_video_dispatcher.cc
+++ b/remoting/protocol/client_video_dispatcher.cc
@@ -66,7 +66,7 @@ void ClientVideoDispatcher::OnPacketDone(
   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();
   }
 }