Bug 1556352 - Part 1: Do not set form owner from parser for form-associated custom element; r=smaug

See steps 14 of https://html.spec.whatwg.org/commit-snapshots/3ad5159be8f27e110a70cefadcb50fc45ec21b05/#create-an-element-for-the-token

From spec perspective, FACE needs this in order to enqueue the formAssociated callback
while FACE is inserted into document. Otherwise it would bail out in
https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#association-of-controls-and-forms:parser-inserted-flag,
and wouldn't run https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#reset-the-form-owner
steps nor https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions:reset-the-form-owner

From implementation perspective, we don't implement parser inserted flag, but we do
update the form owner from parser. Not doing this would make the subsequent part
which implements formAssociated callback a bit simpler, where we don't need to consider
or handle the case that form owner is set from parser.

Differential Revision: https://phabricator.services.mozilla.com/D129646

parser/html/nsHtml5TreeOperation.cpp

@@ -645,6 +645,8 @@ void nsHtml5TreeOperation::SetFormElement(nsIContent* aNode,
                "The form element doesn't implement HTMLFormElement.");
   nsCOMPtr<nsIFormControl> formControl(do_QueryInterface(aNode));
   if (formControl &&
+      formControl->ControlType() !=
+          FormControlType::FormAssociatedCustomElement &&
       !aNode->AsElement()->HasAttr(kNameSpaceID_None, nsGkAtoms::form)) {
     formControl->SetForm(formElement);
   } else if (HTMLImageElement* domImageElement =