MessageEvent constructor should accept a MessagePort as source

MessageEvent constructor should accept a MessagePort as source in the
MessageEventInit dictionary. Previously, it was only accepting a
Window object as source.

The specification was updated recently to address this issue:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=23011
http://html5.org/tools/web-apps-tracker?from=8147&to=8148

R=abarth
BUG=276474

Review URL: https://chromiumcodereview.appspot.com/22915005

git-svn-id: svn://svn.chromium.org/blink/trunk@156420 bbb929c8-8fbe-4397-9dbb-9b2b20218538