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

content/browser/renderer_host/input/input_router_impl.cc

Index: content/browser/renderer_host/input/input_router_impl.cc
diff --git a/content/browser/renderer_host/input/input_router_impl.cc b/content/browser/renderer_host/input/input_router_impl.cc
index e6ddcd9ed7f0604b966f382d39266f4c7ed7f984..184eefe046885d3d0b82e4720f140bcb80d3ca11 100644
--- a/content/browser/renderer_host/input/input_router_impl.cc
+++ b/content/browser/renderer_host/input/input_router_impl.cc
@@ -62,6 +62,12 @@ const char* GetEventAckName(InputEventAckState ack_result) {
   return "";
 }
 
+enum TouchEventDispatchResultType {
+  UNHANDLED_TOUCHES,  //  Unhandled touch events.
+  HANDLED_TOUCHES,    // Handled touch events.
+  TOUCH_EVENT_DISPATCH_RESULT_TYPE_COUNT
+};
+
 } // namespace
 
 InputRouterImpl::Config::Config() {
@@ -454,6 +460,16 @@ void InputRouterImpl::OnInputEventAck(const InputEventAck& ack) {
     OnDidOverscroll(*ack.overscroll);
   }
 
+  // Count the canceled touch starts while there is an active fling animation.
+  if (gesture_event_queue_.FlingInProgress() &&
+      ack.type == WebInputEvent::TouchStart) {
+    TouchEventDispatchResultType result =
+        (ack.state == INPUT_EVENT_ACK_STATE_CONSUMED) ? HANDLED_TOUCHES

[dtapuska, 2016/04/27 19:05:13]

I presume the passive ones counted as unhandled is ok. Do we want to call the
passive ones out to see it increase so we can see the intervention taking place?

+                                                      : UNHANDLED_TOUCHES;
+    UMA_HISTOGRAM_ENUMERATION("Event.Touch.TouchStartsCanceledDuringFling",
+                              result, TOUCH_EVENT_DISPATCH_RESULT_TYPE_COUNT);
+  }
+
   ProcessInputEventAck(ack.type, ack.state, ack.latency,
                        ack.unique_touch_event_id, RENDERER);
 }