Don't pump message loop when sending sync msg

In old days sending sync message to browser process requires pumping
message loop in the renderer process, but now in Chrome 47 it is not
true anymore. And even when we do it, the Send method may fail
sometimes, so this change seems to be required for the Chrome 47
upgrade.

atom/renderer/api/atom_api_renderer_ipc.cc

@@ -54,8 +54,6 @@ base::string16 SendSync(mate::Arguments* args,
 
   IPC::SyncMessage* message = new AtomViewHostMsg_Message_Sync(
       render_view->GetRoutingID(), channel, arguments, &json);
-  // Enable the UI thread in browser to receive messages.
-  message->EnableMessagePumping();
   bool success = render_view->Send(message);
 
   if (!success)