Add extra thread hops in WebUI code on ChromeOS to simulate old timing.

The removal of the UI->IO thread hop in r768133 seems to cause some tests in the PFQ to flake. Add extra hops (now UI->IO->UI) to avoid flakiness in the PFQ. This should be reverted once the test issue is tracked down.

Bug: 1082298

Change-Id: I64e3e27461a73bf269c8193d9d5c7a65cb3a072a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2202671
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Jenny Zhang <jennyz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769037}

content/browser/webui/web_ui_url_loader_factory.cc

@@ -291,8 +291,20 @@ class WebUIURLLoaderFactory : public network::mojom::URLLoaderFactory,
     // from frames can happen while the RFH is changed for a cross-process
     // navigation. The URLDataSources just need the WebContents; the specific
     // frame doesn't matter.
+#if defined(OS_CHROMEOS)
+    // https://crbug.com/1082326 PFQ has test suites that flaked when the thread
+    // hop to IO thread got eliminated. So add an extra IO->UI thread hop to
+    // simulate old timing to avoid this flake for now. This should be fixed to
+    // remove these unnecessary hops.
+    base::PostTaskAndReply(
+        FROM_HERE, {BrowserThread::IO}, base::DoNothing(),
+        base::BindOnce(
+            &StartURLLoader, request, render_frame_host_->GetFrameTreeNodeId(),
+            std::move(client), GetStoragePartition()->browser_context()));
+#else
     StartURLLoader(request, render_frame_host_->GetFrameTreeNodeId(),
                    std::move(client), GetStoragePartition()->browser_context());
+#endif
   }
 
   void Clone(mojo::PendingReceiver<network::mojom::URLLoaderFactory> receiver)