[whatwg/dom] Web reality: DOMTokenList's toggle() cannot always update the corresponding attribute (#451)

@smaug---- mentioned https://bugzilla.mozilla.org/show_bug.cgi?id=1360230 in #444. Which is a little sad, but also fine to fix.

So we basically want to go back to only invoking the "update steps" when we mutate the set first. Unclear if we should do the same for `replace()`. Given this is a special case it probably warrants a note as well.

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