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..4a61e55a20a2fe8bfb27294dc3c69d8176590b88 100644
--- a/public/web/WebDOMMessageEvent.h
+++ b/public/web/WebDOMMessageEvent.h
@@ -47,6 +47,8 @@ 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());

[dcheng, 2015/08/18 16:46:54]

Optional for this patch, but it'd be nice to introduce a class comment on how to
use this class or what it's for? We can stick this into the cleanup patch to
unblock the Chrome-side work.

[sgurun-gerrit only, 2015/08/18 16:49:07]

will add it as part of cleanup.

+    // DEPRECATED: Use the initMessageEvent method above. This method will be removed.
     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;