Add main thread enforcing DCHECK in Dispatcher's messaging IPC handler.

Dispatcher is expected to receive messaging IPCs for main thread and not
worker threads.
Add the check to Dispatcher::OnDeliverMesage, similar to what existed
for other messages, e.g. Dispatcher::OnDispatchOnDisconnect.

Bug: None
Test: No visible changes expected.
Change-Id: I0c11b302b9d9610b1f8792ebf2a385e0aec9fb77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1508601Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638679}

extensions/renderer/dispatcher.cc

@@ -936,6 +936,7 @@ void Dispatcher::OnCancelSuspend(const std::string& extension_id) {
 void Dispatcher::OnDeliverMessage(int worker_thread_id,
                                   const PortId& target_port_id,
                                   const Message& message) {
+  DCHECK_EQ(kMainThreadId, worker_thread_id);
   bindings_system_->GetMessagingService()->DeliverMessage(
       script_context_set_.get(), target_port_id, message,
       NULL);  // All render frames.