Start changing WebDOMMessageEvent for using target execution context.

public/web/WebDOMMessageEvent.h

Index: public/web/WebDOMMessageEvent.h
diff --git a/public/web/WebDOMMessageEvent.h b/public/web/WebDOMMessageEvent.h
index 5d8dc08f73a5d6a729e17d0366f1b32066982eb6..c02a410671acaa7e38354310d463800a430a1511 100644
--- a/public/web/WebDOMMessageEvent.h
+++ b/public/web/WebDOMMessageEvent.h
@@ -1,4 +1,4 @@
-/*
+ /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -47,6 +47,7 @@ class WebString;
 class WebDOMMessageEvent : public WebDOMEvent {
 public:
     WebDOMMessageEvent() { }
+    BLINK_EXPORT void initMessageEvent(const WebString& type, bool canBubble, bool cancelable, const WebSerializedScriptValue& messageData, const WebString& origin, const WebFrame* sourceFrame, const WebDocument& targetDocument, const WebString& lastEventId, const WebMessagePortChannelArray& channels = WebMessagePortChannelArray());

[adamk, 2015/08/15 00:01:16]

You'll need someone in public/OWNERS to review this API change.

     BLINK_EXPORT void initMessageEvent(const WebString& type, bool canBubble, bool cancelable, const WebSerializedScriptValue& messageData, const WebString& origin, const WebFrame* sourceFrame, const WebString& lastEventId, const WebMessagePortChannelArray& channels = WebMessagePortChannelArray());
 
     BLINK_EXPORT WebSerializedScriptValue data() const;