Add UMA metric for tracking root level listeners for blocking touch.

third_party/WebKit/Source/core/events/TouchEvent.h

Index: third_party/WebKit/Source/core/events/TouchEvent.h
diff --git a/third_party/WebKit/Source/core/events/TouchEvent.h b/third_party/WebKit/Source/core/events/TouchEvent.h
index 4bb598dc6c385cf614f029f82e7e3d20ac94974e..381b9ba927584bee7184112a705d45e2cd643918 100644
--- a/third_party/WebKit/Source/core/events/TouchEvent.h
+++ b/third_party/WebKit/Source/core/events/TouchEvent.h
@@ -82,6 +82,8 @@ public:
 
     void preventDefault() override;
 
+    void doneDispatchingEventAtCurrentTarget() override;
+
     EventDispatchMediator* createMediator() override;
 
     DECLARE_VIRTUAL_TRACE();
@@ -99,6 +101,7 @@ private:
     Member<TouchList> m_targetTouches;
     Member<TouchList> m_changedTouches;
     bool m_causesScrollingIfUncanceled;
+    bool m_defaultPreventedBeforeCurrentTarget;
 };
 
 class TouchEventDispatchMediator final : public EventDispatchMediator {