UMA metrics for fractions of wheel and touch scrolls blocked on the main thread.

ui/events/blink/input_handler_proxy.h

Index: ui/events/blink/input_handler_proxy.h
diff --git a/ui/events/blink/input_handler_proxy.h b/ui/events/blink/input_handler_proxy.h
index 6f86d22ad5378782323b8d32429af0fddef7c3ea..59277bbed7a34b3a3ffda6dca355b4096f3ceaf8 100644
--- a/ui/events/blink/input_handler_proxy.h
+++ b/ui/events/blink/input_handler_proxy.h
@@ -117,6 +117,8 @@ class InputHandlerProxy
  protected:
   void RecordMainThreadScrollingReasons(blink::WebGestureDevice device,
                                         uint32_t reasons);
+  void RecordScrollingThreadStatus(blink::WebGestureDevice device,
+                                   uint32_t reasons);
 
  private:
   friend class test::InputHandlerProxyTest;
@@ -244,6 +246,10 @@ class InputHandlerProxy
   // passive events not to block scrolling.
   int32_t touch_start_result_;
 
+  // The result of the last mouse wheel event. This value is used to determine
+  // whether the next wheel scroll is blocked on the Main thread or not.
+  int32_t mouse_wheel_result_;
+
   base::TimeTicks last_fling_animate_time_;
 
   // Used to record overscroll notifications while an event is being