Add a BeginMainFrame fallback to MainThreadEventQueue.

content/renderer/input/main_thread_event_queue.h

Index: content/renderer/input/main_thread_event_queue.h
diff --git a/content/renderer/input/main_thread_event_queue.h b/content/renderer/input/main_thread_event_queue.h
index 5001f046fd07f24b6ffa4b14419d12e0e52dacb9..789081be1cfeb7bc210d047854baff58d166ad1b 100644
--- a/content/renderer/input/main_thread_event_queue.h
+++ b/content/renderer/input/main_thread_event_queue.h
@@ -118,6 +118,7 @@ class CONTENT_EXPORT MainThreadEventQueue
   bool IsRafAlignedInputDisabled() const;
   bool IsRafAlignedEvent(
       const std::unique_ptr<MainThreadEventQueueTask>& item) const;
+  void RafFallbackTimerFired();
 
   friend class QueuedWebInputEvent;
   friend class MainThreadEventQueueTest;
@@ -149,6 +150,7 @@ class CONTENT_EXPORT MainThreadEventQueue
 
   scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
   blink::scheduler::RendererScheduler* renderer_scheduler_;
+  base::OneShotTimer raf_fallback_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(MainThreadEventQueue);
 };