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 - HTMLElement.prototype.attachInternals() should throw InvalidStateErrors
   for customized built-in elements.
 - Form-related operations and attributes of ElementInternals should throw
   InvalidStateErrors for non-form-associated custom elements.

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: I09e574a843296b236fe4c63eb2cd0912c7a673b1
Reviewed-on: https://chromium-review.googlesource.com/c/1353035Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611960}