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

ui/events/blink/input_handler_proxy.cc

 // 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.
 
 #include "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 757 matching lines @@
       return DID_HANDLE;
     }
     case cc::InputHandler::SCROLL_UNKNOWN:
     case cc::InputHandler::SCROLL_ON_MAIN_THREAD: {
       TRACE_EVENT_INSTANT0("input",
                            "InputHandlerProxy::HandleGestureFling::"
                            "scroll_on_main_thread",
                            TRACE_EVENT_SCOPE_THREAD);
       gesture_scroll_on_impl_thread_ = false;
       fling_may_be_active_on_main_thread_ = true;
+      client_->DidStartFlinging();
       return DID_NOT_HANDLE;
     }
     case cc::InputHandler::SCROLL_IGNORED: {
       TRACE_EVENT_INSTANT0(
           "input",
           "InputHandlerProxy::HandleGestureFling::ignored",
           TRACE_EVENT_SCOPE_THREAD);
       gesture_scroll_on_impl_thread_ = false;
       if (gesture_event.sourceDevice == blink::WebGestureDeviceTouchpad) {
         // We still pass the curve to the main thread if there's nothing
@@ skipping 478 matching lines @@
                            "InputHandlerProxy::scrollBy::AbortFling",
                            TRACE_EVENT_SCOPE_THREAD);
       // If we got a DID_NOT_HANDLE, that means we need to deliver wheels on the
       // main thread. In this case we need to schedule a commit and transfer the
       // fling curve over to the main thread and run the rest of the wheels from
       // there. This can happen when flinging a page that contains a scrollable
       // subarea that we can't scroll on the thread if the fling starts outside
       // the subarea but then is flung "under" the pointer.
       client_->TransferActiveWheelFlingAnimation(fling_parameters_);
       fling_may_be_active_on_main_thread_ = true;
+      client_->DidStartFlinging();
       CancelCurrentFlingWithoutNotifyingClient();
       break;
   }
 
   return false;
 }
 
 bool InputHandlerProxy::scrollBy(const WebFloatSize& increment,
                                  const WebFloatSize& velocity) {
   WebFloatSize clipped_increment;
@@ skipping 70 matching lines @@
   // is made asynchronously, to minimize divergence between main thread and
   // impl thread event handling paths.
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(&InputScrollElasticityController::ObserveGestureEventAndResult,
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 }  // namespace ui