[Blimp Net] Use checkpoitner

Otherwise, ACK buffer will buildup in BlimpMessageOutputBuffer.

Review URL: https://codereview.chromium.org/1643813002

Cr-Commit-Position: refs/heads/master@{#371966}

blimp/net/browser_connection_handler.cc

@@ -51,7 +51,7 @@ void BrowserConnectionHandler::HandleConnection(
   connection_ = std::move(connection);
 
   // Hook up message streams to the connection.
-  connection_->SetIncomingMessageProcessor(demultiplexer_.get());
+  connection_->SetIncomingMessageProcessor(checkpointer_.get());
   output_buffer_->SetOutputProcessor(
       connection_->GetOutgoingMessageProcessor());
   connection_->AddConnectionErrorObserver(this);