Add UMA metric for tracking listeners for blocking touch while fling is happening

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 e047cdfe10a4dbacdf5ba5d492504a905d2ae172..f5dd15fb71fa9f59a78d0a68be55689cd1b52553 100644
--- a/ui/events/blink/input_handler_proxy.h
+++ b/ui/events/blink/input_handler_proxy.h
@@ -92,6 +92,10 @@ class InputHandlerProxy
     return gesture_scroll_on_impl_thread_;
   }
 
+  bool isFlinging() const {
+    return (!!fling_curve_) || fling_may_be_active_on_main_thread_;
+  }
+
  protected:
   void RecordMainThreadScrollingReasons(blink::WebInputEvent::Type type,
                                         uint32_t reasons);