Simplify data channel creation logic in WebRTC-based protocol

remoting/protocol/message_pipe.h

 // Copyright 2016 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.
 
 #ifndef REMOTING_PROTOCOL_MESSAGE_PIPE_H_
 #define REMOTING_PROTOCOL_MESSAGE_PIPE_H_
 
 #include <memory>
 
 #include "base/callback_forward.h"
 
 namespace google {
 namespace protobuf {
 class MessageLite;
 }  // namespace protobuf
 }  // namespace google
 
 namespace remoting {
 
 class CompoundBuffer;
 
 namespace protocol {
 
 // Represents a bi-directional pipe that allows to send and receive messages.
 class MessagePipe {
  public:
   class EventHandler {
    public:
+    // Called when the channel is open.
+    virtual void OnMessagePipeOpen() = 0;
+
     // Called when a message is received.
     virtual void OnMessageReceived(std::unique_ptr<CompoundBuffer> message) = 0;
 
     // Called when the channel is closed.
     virtual void OnMessagePipeClosed() = 0;
 
    protected:
     virtual ~EventHandler() {}
   };
 
   virtual ~MessagePipe() {}
 
-  // Starts the channel. |event_handler| will be called to notify when a message
-  // is received or the pipe is closed.
+  // Starts the channel. Must be called immediately after MessagePipe is
+  // created. |event_handler| will be notified when state of the pipe changes or
+  // when a message is received.
   virtual void Start(EventHandler* event_handler) = 0;
 
   // Sends a message. |done| is called when the message has been sent to the
   // client, but it doesn't mean that the client has received it. |done| is
   // never called if the message is never sent (e.g. if the pipe is destroyed
   // before the message is sent).
   virtual void Send(google::protobuf::MessageLite* message,
                     const base::Closure& done) = 0;
 };
 
 }  // namespace protocol
 }  // namespace remoting
 
 #endif  // REMOTING_PROTOCOL_MESSAGE_PIPE_H_