Add UMA metrics for root scroller intervention to track forcing passive breakage.

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

Index: third_party/WebKit/Source/core/events/Event.h
diff --git a/third_party/WebKit/Source/core/events/Event.h b/third_party/WebKit/Source/core/events/Event.h
index 119b255c03dffa8aad48cc4d6fc21adbc388c25d..c53054f03309c3169a1e2250d9c06db946785a7c 100644
--- a/third_party/WebKit/Source/core/events/Event.h
+++ b/third_party/WebKit/Source/core/events/Event.h
@@ -214,7 +214,9 @@ public:
     bool isTrusted() const { return m_isTrusted; }
     void setTrusted(bool value) { m_isTrusted = value; }
 
-    void setHandlingPassive(bool value) { m_handlingPassive = value; }
+    void setHandlingPassive(bool value);
+
+    bool preventDefaultCalledDuringPassive() const { return m_preventDefaultCalledDuringPassive; }
 
     DECLARE_VIRTUAL_TRACE();
 
@@ -253,6 +255,10 @@ private:
     unsigned m_isTrusted : 1;
     unsigned m_handlingPassive : 1;
 
+    // Whether preventDefault was called when |m_handlingPassive| is
+    // true. This field is reset on each call to setHandlingPassive.
+    unsigned m_preventDefaultCalledDuringPassive : 1;
+
     unsigned short m_eventPhase;
     Member<EventTarget> m_currentTarget;
     Member<EventTarget> m_target;