Use ASCII serialization for origins (#1151)
This patch makes it to use ASCII serialization for
(Extendable)MessageEvent.origin instead of Unicode serialization which
has been removed from HTML.
HTML issue: https://github.com/whatwg/html/issues/2568.
HTML change: https://github.com/whatwg/html/pull/2689/commits/c3b2511ee0158fa4b5bd4e4689c6be54132f3d90.
Fixes #1142.
