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

ui/events/blink/input_handler_proxy_client.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 #define UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_
 
 namespace blink {
 class WebGestureCurve;
 struct WebActiveWheelFlingParameters;
@@ skipping 20 matching lines @@
       blink::WebGestureDevice device_source,
       const blink::WebFloatPoint& velocity,
       const blink::WebSize& cumulative_scroll) = 0;
 
   virtual void DidOverscroll(
       const gfx::Vector2dF& accumulated_overscroll,
       const gfx::Vector2dF& latest_overscroll_delta,
       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;
 
  protected:
   virtual ~InputHandlerProxyClient() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_BLINK_INPUT_HANDLER_PROXY_CLIENT_H_