Origin API: need to change MessageEvent

[annevk]

What is the issue with the HTML Standard?

@mikewest I missed this during review, but we need to change MessageEvent (and presumably ExtendableMessageEvent) so they also store the actual origin as an internal field.

[annevk]

Also, wouldn't it be better if we end up throwing for the cases where there is no true backing origin? When MessageEvent's origin is created from a string or when initMessageEvent() is called it seems that Origin.from() shouldn't work.