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..966adb5bdcfc9712671f73dff2c11f4b7cb96592 100644
--- a/ui/events/blink/input_handler_proxy_client.h
+++ b/ui/events/blink/input_handler_proxy_client.h
@@ -42,6 +42,8 @@ class InputHandlerProxyClient {
 
   virtual void DidAnimateForInput() = 0;
 
+  virtual void DidStartFlinging(bool is_on_impl) = 0;
+
  protected:
   virtual ~InputHandlerProxyClient() {}
 };