Bug 1342258 - Refactor SetDomain to IsRegistrableDomainSuffixOfOrEqua…

…lTo r=smaug

This commit refactors the SetDomain method in a Document to call a new function
IsRegistrableDomainSuffixOfOrEqualTo(), defined in HTML [1]. This commit tries
not to rename anything except input variables, so as to remain as clear as
possible. It likely should have various variables renamed, but given the
author's unfamiliarity with this module, review seems a good time to do that.
It's also duplicating comments a little bit; let me know which one(s) you'd like
to keep!

Note: Commentary on the HTML change is available in the PR [2], and the
rationale for this behavior in Web Auentication, where this algorithm will be
used, is also recorded [3].

Updates: Refactored two new protected methods to avoid code duplication.

[1] https://html.spec.whatwg.org/multipage/browsers.html#is-a-registrable-domain-suffix-of-or-is-equal-to
[2] whatwg/html#2365
[3] w3ctag/design-reviews#97 (comment)

MozReview-Commit-ID: 4Dr8yOMdhez

UltraBlame original commit: ffa2f50d49ce0356bbf02ea0940060283dcc007c