document.open()'s mutation event suppression

[annevk]

document.open(), and various other places, such as the HTML parser, suppress mutation events. Unfortunately (and per usual) it differs across browsers. Chrome/Edge/Safari appear to set a one-off flag, whereas Firefox will only suppress them between document.open() and document.close().

That is, if you add frame.contentDocument.body.appendChild(document.createElement("x")); after the close() call in web-platform-tests/wpt#10680, Firefox will "fail" the test.

If we ever define them in detail, see #305, we should make sure this nuance is covered.