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

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

Index: third_party/WebKit/Source/core/input/TouchEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/TouchEventManager.cpp b/third_party/WebKit/Source/core/input/TouchEventManager.cpp
index 5fbb1139f481d6dcf3bea6dfe89f759b3deb01d4..204da5140c6bc9db5b09967e5bfc80ee94d90d55 100644
--- a/third_party/WebKit/Source/core/input/TouchEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/TouchEventManager.cpp
@@ -251,6 +251,12 @@ WebInputEventResult TouchEventManager::dispatchTouchEvents(
                     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.shouldForceBePassive()) {
+                    DEFINE_STATIC_LOCAL(EnumerationHistogram, touchDispositionsDuringFlingHistogram, ("Event.Touch.TouchDispositionsDuringFling", TouchEventDispatchResultTypeMax));
+                    touchDispositionsDuringFlingHistogram.count((domDispatchResult != DispatchEventResult::NotCanceled) ? HandledTouches : UnhandledTouches);
+                }
             }
             eventResult = EventHandler::mergeEventResult(eventResult,
                 EventHandler::toWebInputEventResult(domDispatchResult));