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

ui/events/blink/input_handler_proxy_client.h

Index: ui/events/blink/input_handler_proxy_client.h
diff --git a/ui/events/blink/input_handler_proxy_client.h b/ui/events/blink/input_handler_proxy_client.h
index 34ce9fe95a9b4a1a2b7ed2c9dfc13de13309e3af..efc24ed6d17d4a83b8879422c96db83481f7d89a 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -38,6 +38,8 @@ class InputHandlerProxyClient {
       const gfx::Vector2dF& current_fling_velocity,
       const gfx::PointF& causal_event_viewport_point) = 0;
 
+  virtual void DidStartFlinging() = 0;
+
   virtual void DidStopFlinging() = 0;
 
   virtual void DidAnimateForInput() = 0;