[dom] DOMTokenList hook to validate added token is in a supported set of keywords (#103)

This is a stab at adding the required hook in DOM so that `iframe@sandbox`, `link@rel` and hopefully future features would be able to use `DOMTokenList.add()` for feature detection.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29061

/cc @annevk @domenic @zcorpan @Ms2ger 
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/103

-- Commit Summary --

  * Add DOMTokenList hook to validate added token is in a supported set of keywords.

-- File Changes --

    M dom.bs (23)
    M dom.html (29)

-- Patch Links --

https://github.com/whatwg/dom/pull/103.patch
https://github.com/whatwg/dom/pull/103.diff

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/pull/103