Add MessagePipe interface. Use it in ChannelDispatcherBase.

remoting/protocol/webrtc_connection_to_client.cc

Index: remoting/protocol/webrtc_connection_to_client.cc
diff --git a/remoting/protocol/webrtc_connection_to_client.cc b/remoting/protocol/webrtc_connection_to_client.cc
index 1c23e2ddba0d17b069aa1fe1faa0b98ce278c28c..e84513723983d6cd8a1689aa2f049dfeaae7cb03 100644
--- a/remoting/protocol/webrtc_connection_to_client.cc
+++ b/remoting/protocol/webrtc_connection_to_client.cc
@@ -149,7 +149,6 @@ void WebrtcConnectionToClient::OnWebrtcTransportConnecting() {
 
 void WebrtcConnectionToClient::OnWebrtcTransportConnected() {
   DCHECK(thread_checker_.CalledOnValidThread());
-  event_handler_->OnConnectionChannelsConnected(this);
 }
 
 void WebrtcConnectionToClient::OnWebrtcTransportError(ErrorCode error) {
@@ -168,6 +167,11 @@ void WebrtcConnectionToClient::OnWebrtcTransportMediaStreamRemoved(
 void WebrtcConnectionToClient::OnChannelInitialized(
     ChannelDispatcherBase* channel_dispatcher) {
   DCHECK(thread_checker_.CalledOnValidThread());
+
+  if (control_dispatcher_ && control_dispatcher_->is_connected() &&
+      event_dispatcher_ && event_dispatcher_->is_connected()) {
+    event_handler_->OnConnectionChannelsConnected(this);
+  }
 }
 
 void WebrtcConnectionToClient::OnChannelError(