Re: [whatwg/dom] Should attachShadow throw or override when an existing declarative shadow root doesn't match? (Issue #1235)

> I suppose that could work, presuming you mean _always_ ignore subsequent declarative shadow roots, not just when their parameters mismatch. It's a change in behavior, so it might not be web compatible, but it seems relatively safe. If it sounds like you're happy with that, I'll try to implement it and see what happens.

Hearing no objections, I went ahead and implemented this in Chromium, and changed/added some WPTs. I also put up a corresponding set of [HTML](https://github.com/whatwg/html/pull/10069)/[DOM](https://github.com/whatwg/dom/pull/1246) spec PRs. To summarize the changes:

 1. If two declarative shadow roots are included within a single host element, only the *first* one will remain. This is a change in behavior from before, but it hopefully won't encounter compat issues (TBD).
 2. If attachShadow() is used on an existing declarative shadow root, and the parameters (e.g. shadow root type, delegates focus, etc.) do not match, an exception is thrown. This, also, is a breaking change, but again hopefully not one that has compat implications.

Hopefully folks are happy with these changes. If so, I'll try shipping them in the next Chrome and verify the compat safety.

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