Add MessagePipe interface. Use it in ChannelDispatcherBase.

remoting/protocol/host_control_dispatcher.cc

Index: remoting/protocol/host_control_dispatcher.cc
diff --git a/remoting/protocol/host_control_dispatcher.cc b/remoting/protocol/host_control_dispatcher.cc
index fb8aee94188bc111745ec09d893eef012fd2c32e..6b4458a132e7f199d5a75f9fae3cfdb643be79b1 100644
--- a/remoting/protocol/host_control_dispatcher.cc
+++ b/remoting/protocol/host_control_dispatcher.cc
@@ -24,34 +24,34 @@ void HostControlDispatcher::SetCapabilities(
     const Capabilities& capabilities) {
   ControlMessage message;
   message.mutable_capabilities()->CopyFrom(capabilities);
-  writer()->Write(SerializeAndFrameMessage(message), base::Closure());
+  message_pipe()->Send(&message, base::Closure());
 }
 
 void HostControlDispatcher::SetPairingResponse(
     const PairingResponse& pairing_response) {
   ControlMessage message;
   message.mutable_pairing_response()->CopyFrom(pairing_response);
-  writer()->Write(SerializeAndFrameMessage(message), base::Closure());
+  message_pipe()->Send(&message, base::Closure());
 }
 
 void HostControlDispatcher::DeliverHostMessage(
     const ExtensionMessage& message) {
   ControlMessage control_message;
   control_message.mutable_extension_message()->CopyFrom(message);
-  writer()->Write(SerializeAndFrameMessage(control_message), base::Closure());
+  message_pipe()->Send(&control_message, base::Closure());
 }
 
 void HostControlDispatcher::InjectClipboardEvent(const ClipboardEvent& event) {
   ControlMessage message;
   message.mutable_clipboard_event()->CopyFrom(event);
-  writer()->Write(SerializeAndFrameMessage(message), base::Closure());
+  message_pipe()->Send(&message, base::Closure());
 }
 
 void HostControlDispatcher::SetCursorShape(
     const CursorShapeInfo& cursor_shape) {
   ControlMessage message;
   message.mutable_cursor_shape()->CopyFrom(cursor_shape);
-  writer()->Write(SerializeAndFrameMessage(message), base::Closure());
+  message_pipe()->Send(&message, base::Closure());
 }
 
 void HostControlDispatcher::OnIncomingMessage(