InitEvent should not do anything if dispatch flag is set.

third_party/WebKit/Source/core/events/MutationEvent.cpp

Index: third_party/WebKit/Source/core/events/MutationEvent.cpp
diff --git a/third_party/WebKit/Source/core/events/MutationEvent.cpp b/third_party/WebKit/Source/core/events/MutationEvent.cpp
index 1a9b9179877933c88e6040f9440735a4ddd9a212..065e9c0f29a417156d0b316812c908ab8e724590 100644
--- a/third_party/WebKit/Source/core/events/MutationEvent.cpp
+++ b/third_party/WebKit/Source/core/events/MutationEvent.cpp
@@ -49,7 +49,7 @@ void MutationEvent::initMutationEvent(const AtomicString& type, bool canBubble,
                                       const String& prevValue, const String& newValue,
                                       const String& attrName, unsigned short attrChange)
 {
-    if (dispatched())
+    if (isBeingDispatched())
         return;
 
     initEvent(type, canBubble, cancelable);