InitEvent should not do anything if dispatch flag is set.

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

 /*
  * Copyright (C) 2007 Henry Mason (hmason@mac.com)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 122 matching lines @@
 {
     if (initializer.source() && !isValidSource(initializer.source())) {
         exceptionState.throwTypeError("The optional 'source' property is neither a Window nor MessagePort.");
         return nullptr;
     }
     return new MessageEvent(type, initializer);
 }
 
 void MessageEvent::initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, ScriptValue data, const String& origin, const String& lastEventId, DOMWindow* source, MessagePortArray* ports)
 {
-    if (dispatched())
+    if (isBeingDispatched())
         return;
 
     initEvent(type, canBubble, cancelable);
 
     m_dataType = DataTypeScriptValue;
     m_dataAsScriptValue = data;
     m_origin = origin;
     m_lastEventId = lastEventId;
     m_source = source;
     m_ports = ports;
     m_suborigin = "";
 }
 
 void MessageEvent::initMessageEvent(const AtomicString& type, bool canBubble, bool cancelable, PassRefPtr<SerializedScriptValue> data, const String& origin, const String& lastEventId, DOMWindow* source, MessagePortArray* ports)
 {
-    if (dispatched())
+    if (isBeingDispatched())
         return;
 
     initEvent(type, canBubble, cancelable);
 
     m_dataType = DataTypeSerializedScriptValue;
     m_dataAsSerializedScriptValue = data;
     m_origin = origin;
     m_lastEventId = lastEventId;
     m_source = source;
     m_ports = ports;
@@ skipping 60 matching lines @@
         break;
     case MessageEvent::DataTypeArrayBuffer:
         V8HiddenValue::setHiddenValue(ScriptState::current(isolate), wrapper, V8HiddenValue::arrayBufferData(isolate), toV8(dataAsArrayBuffer(), wrapper, isolate));
         break;
     }
 
     return wrapper;
 }
 
 } // namespace blink