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

ui/events/blink/input_handler_proxy_unittest.cc

Index: ui/events/blink/input_handler_proxy_unittest.cc
diff --git a/ui/events/blink/input_handler_proxy_unittest.cc b/ui/events/blink/input_handler_proxy_unittest.cc
index 86cf66250c85ba43faf54cdb3275a9afe0fae3f8..17897f8653ef444f6b6f2dab064248cc99a9dc58 100644
--- a/ui/events/blink/input_handler_proxy_unittest.cc
+++ b/ui/events/blink/input_handler_proxy_unittest.cc
@@ -223,6 +223,7 @@ class MockInputHandlerProxyClient
                     const gfx::PointF& causal_event_viewport_point));
   void DidStopFlinging() override {}
   void DidAnimateForInput() override {}
+  void DidStartFlinging(bool is_on_impl) override {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MockInputHandlerProxyClient);