Add CancelChannelCreation() in protocol::Session interface.

remoting/protocol/connection_to_client.cc

Index: remoting/protocol/connection_to_client.cc
diff --git a/remoting/protocol/connection_to_client.cc b/remoting/protocol/connection_to_client.cc
index 7579463ffc357938f33af88da327bb715423bb0f..d31165fa79537616562eeb6460e52b31da562f3e 100644
--- a/remoting/protocol/connection_to_client.cc
+++ b/remoting/protocol/connection_to_client.cc
@@ -162,10 +162,9 @@ void ConnectionToClient::CloseOnError() {
 }
 
 void ConnectionToClient::CloseChannels() {
-  if (video_writer_.get())
-    video_writer_->Close();
-  if (client_control_sender_.get())
-    client_control_sender_->Close();
+  video_writer_.reset();
+  client_control_sender_.reset();
+  dispatcher_.reset();
 }
 
 }  // namespace protocol