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

third_party/WebKit/Source/core/input/EventHandler.cpp

Index: third_party/WebKit/Source/core/input/EventHandler.cpp
diff --git a/third_party/WebKit/Source/core/input/EventHandler.cpp b/third_party/WebKit/Source/core/input/EventHandler.cpp
index 5f643cf8b2ad4e8a6e7efcf500ca41fed94410ee..828b6bb841b4279a32d4b397f7f3ab9f1f9c70b9 100644
--- a/third_party/WebKit/Source/core/input/EventHandler.cpp
+++ b/third_party/WebKit/Source/core/input/EventHandler.cpp
@@ -3886,6 +3886,12 @@ WebInputEventResult EventHandler::dispatchTouchEvents(const PlatformTouchEvent&
                     DEFINE_STATIC_LOCAL(EnumerationHistogram, touchDispositionsBeforePageLoadHistogram, ("Event.Touch.TouchDispositionsBeforePageLoad", TouchEventDispatchResultTypeMax));
                     touchDispositionsBeforePageLoadHistogram.count((domDispatchResult != DispatchEventResult::NotCanceled) ? HandledTouches : UnhandledTouches);
                 }
+
+                // Count the handled touch starts while there is an active fling animation.
+                if (event.type() == PlatformEvent::TouchStart && event.isFlingInProgress()) {
+                    DEFINE_STATIC_LOCAL(EnumerationHistogram, touchDispositionsDuringFlingHistogram, ("Event.Touch.TouchDispositionsDuringFling", TouchEventDispatchResultTypeMax));
+                    touchDispositionsDuringFlingHistogram.count((domDispatchResult != DispatchEventResult::NotCanceled) ? HandledTouches : UnhandledTouches);
+                }
             }
             eventResult = mergeEventResult(eventResult, toWebInputEventResult(domDispatchResult));
         }