OffMT service worker startup: Create EmbeddedWorkerInstanceClient on the IO thread

Post a task to the IO thread to create EmbeddedWorkerInstanceClient
when off-the-main-thread service worker startup is enabled.

This is a tentative workaround to avoid some race conditions which
happen when we make EmbeddedWorkerInstanceClient independent from
RenderThreadImpl (see crbug.com/995134). Ideally we shouldn't need
to post a task. Instead we should bind EmbeddedWorkerInstanceClient
on the IO thread.

Bug: 538751
Change-Id: I4bdb54232fb7abde7d6d1dccf1ac3cb58db16a92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761888Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688436}

content/renderer/render_thread_impl.cc

@@ -2090,9 +2090,30 @@ void RenderThreadImpl::CreateFrameProxy(
       replicated_state, devtools_frame_token);
 }
 
+void StartEmbeddedWorkerInstanceClientOnIOThread(
+    mojo::PendingReceiver<blink::mojom::EmbeddedWorkerInstanceClient>
+        client_receiver,
+    scoped_refptr<base::SingleThreadTaskRunner> io_task_runner) {
+  DCHECK(io_task_runner->BelongsToCurrentThread());
+  EmbeddedWorkerInstanceClientImpl::Create(std::move(client_receiver),
+                                           std::move(io_task_runner));
+}
+
 void RenderThreadImpl::SetUpEmbeddedWorkerChannelForServiceWorker(
     mojo::PendingReceiver<blink::mojom::EmbeddedWorkerInstanceClient>
         client_receiver) {
+  // TODO(bashi): This is a tentative workaround to start service worker on the
+  // IO thread. We should decouple EmbeddedWorkerInstanceClient from Renderer
+  // and bind EmbeddedWorkerInstanceClient on the IO thread.
+  if (base::FeatureList::IsEnabled(
+          blink::features::kOffMainThreadServiceWorkerStartup)) {
+    GetIOTaskRunner()->PostTask(
+        FROM_HERE,
+        base::BindOnce(&StartEmbeddedWorkerInstanceClientOnIOThread,
+                       std::move(client_receiver), GetIOTaskRunner()));
+    return;
+  }
+
   EmbeddedWorkerInstanceClientImpl::Create(
       std::move(client_receiver),
       GetWebMainThreadScheduler()->DefaultTaskRunner());