[dom] Design base URL change notification system (#61)

Per https://www.w3.org/Bugs/Public/show_bug.cgi?id=20976#c6 base URL will become a concept of documents.

Then we need a way to set the base URL of a document from another specification. When set, this changes the URL of the document and then for each inclusive descendant, in tree order, notifies the node of a base URL change, if the node registered for base URL changes. This allows e.g., `<a>` to correct its URL.

We also change adopt to run these base URL change notifications. It's unclear whether they should run before the adoption notification or after, or whether the adoption notification by itself is sufficient. I suspect we want to have both separately so you don't forget any.

@ajklein @coonsta @smaug---- does this make sense to you?

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https://github.com/whatwg/dom/issues/61