Re: [whatwg/dom] Un-merge Document and HTMLDocument (#221)

I did a little bit of research with `SELECT page,url FROM [httparchive:har.2016_08_01_chrome_requests_bodies] WHERE body CONTAINS 'instanceof XMLDocument'` in BigQuery:
* [htmldocument.csv](https://gist.github.com/foolip/5109c805a6cd5eb1117607da17b55582) (used `WHERE RAND() < 1/10` to get about 10%, leaving 1254 rows)
* [xmldocument.csv](https://gist.github.com/foolip/cd6b62f1b9244a4c29d3e1d49f1e41a1) (76 rows)

It's immediately clear that `instaceof HTMLDocument` is way more common. I randomized the order and the first thing I found came from mutation-summary.js by @rafaelw:
https://github.com/rafaelw/mutation-summary/blob/421110f84178aa9e4098b38df83f727e5aea3d97/src/mutation-summary.js#L112

@rafaelw, I didn't look too closely, but I take it this code actually matters for getting the correct behavior in both XML and HTML documents?

Copies of this code seem to dominate the usage. http://cdn.clicktale.net/www/ChangeMonitor-latest.js showed up a lot but seems to include an older version of mutation-summary.

Given the volume of results, I'd be cautious about changing the truthyness of `instanceof HTMLDocument` without further analysis.

As for XMLDocument, after checking the most common URL patterns I couldn't find anything that seemed like it would break. So I'd give better odds to making XMLDocument an alias of Document, but that's not a good idea as long as Gecko needs `XMLDocument.protoype.load`.

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