Added missing MainThreadScrollingReason in histograms.xml

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 32c0efc25311e463777f1c2bdc0bdcc0e21d0b4f..ef3ca62ae12a3d638279b0c3cec448e5ed993a0c 100644
--- a/ui/events/blink/input_handler_proxy.h
+++ b/ui/events/blink/input_handler_proxy.h
@@ -89,6 +89,10 @@ class InputHandlerProxy
     return gesture_scroll_on_impl_thread_;
   }
 
+ protected:
+  void RecordMainThreadScrollingReasons(blink::WebInputEvent::Type type,
+                                        uint32_t reasons);
+
  private:
   friend class test::InputHandlerProxyTest;