Update worker thread priority when doing RAF

When doing RAF, Worker thread priority changes to DISPLAY.

Bug: 995235
Change-Id: I44479a6c0c918c24996776cdb1619993d6cd9ddf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796482
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695670}

third_party/blink/renderer/platform/graphics/begin_frame_provider.cc

@@ -59,6 +59,10 @@ void BeginFrameProvider::CreateCompositorFrameSinkIfNeeded() {
   if (compositor_frame_sink_.is_bound())
     return;
 
+  // Once we are using RAF, this thread is driving Display updates. Update
+  // priority accordingly.
+  base::PlatformThread::SetCurrentThreadPriority(base::ThreadPriority::DISPLAY);
+
   mojo::Remote<mojom::blink::EmbeddedFrameSinkProvider> provider;
   Platform::Current()->GetInterfaceProvider()->GetInterface(
       provider.BindNewPipeAndPassReceiver());