synchronously use same origin-domain in iframe check

[marcoscaceres]

Closes #106

Now checks "same origin-domain" instead of the more restrictive "same origin".

This change:

• Adds new normative requirements

Implementation commitment (delete if not making normative changes):

• [ ] Modified Web platform tests (link)
• [ ] WebKit
• [ ] Chromium (https://bugs.chromium.org/)
• [ ] Gecko (http://bugzilla.mozilla.org)

Person merging, please make sure that commits are squashed with one of the following as a commit message prefix:

• chore:
• editorial:
• BREAKING CHANGE:
• And use none if it's a normative change

[marcoscaceres]

Updated WebKit: https://github.com/WebKit/WebKit/pull/13389

I'll update the WPTests also to add a specific same origin-domain check.