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..4fa9326b6c19829cbc1be2966c19f134d8bea266 100644
--- a/ui/events/blink/input_handler_proxy_unittest.cc
+++ b/ui/events/blink/input_handler_proxy_unittest.cc
@@ -221,6 +221,7 @@ class MockInputHandlerProxyClient
                     const gfx::Vector2dF& latest_overscroll_delta,
                     const gfx::Vector2dF& current_fling_velocity,
                     const gfx::PointF& causal_event_viewport_point));
+  void DidStartFlinging() override {}
   void DidStopFlinging() override {}
   void DidAnimateForInput() override {}