[webmessaging-tests] add initMessageEvent() tests for MessageEvent (#1669)

initMessageEvent()  method is part of the prototype of the MessageEvent
interface (https://html.spec.whatwg.org/multipage/comms.html#the-messageevent-interfaces). It's implemented in SF, Ch, IE and Op. Firefox removed the
initMessageEvent function from FF26 (https://code.google.com/p/dart/issues/detail?id=15651).

View on GitHub: https://github.com/w3c/web-platform-tests/pull/1669

Received on Tuesday, 10 March 2015 11:13:47 UTC