appendChild isn't upgrading a custom element

[pshaughn]

WPT custom-elements/Document-createElement.html creates an element with a 'my-div' is, then registers 'my-div', then appends the element, and the element has not become an instance of the registered class.

[pshaughn]

There are a lot of test cases; this one is the assert_true with 'Undefined element is upgraded on connecting to a document'

[pshaughn]

get_is is returning None on the element in question, so try_upgrade_element doesn't proceed to enqueue an upgrade. Possibly this has something in common with #25008 regarding the handling of ElementCreationOptions.

[pshaughn]

Adding to the end of Document::CreateElement like so:

	warn!("Creating {:?} element with is {:?}",name,is);
        let el=Element::create(
            name,
            is,
            self,
            ElementCreator::ScriptCreated,
            CustomElementCreationMode::Synchronous,
        );
	warn!("Created element with is {:?}",el.get_is());
	Ok(el)

I observe:

 0:11.68 pid:18583 [2019-12-23T00:46:20Z WARN  script::dom::document] Creating QualName { prefix: None, ns: Atom('http://www.w3.org/1999/xhtml' type=static), local: Atom('div' type=static) } element with is Some(Atom('my-div' type=inline))
 0:11.68 pid:18583 [2019-12-23T00:46:20Z WARN  script::dom::document] Created element with is None

[pshaughn]

I think I have it pinned down now, https://dom.spec.whatwg.org/#concept-create-element uses the is value as a string in 7.2 even when there's not currently a definition on the name, but

servo/components/script/dom/create.rs

Lines 192 to 193 in 43a5f65

	// Steps 7.1-7.2
	let result = create_native_html_element(name.clone(), prefix, document, creator);

does not. It looks like just adding a set_is right after create_native_html_element will clear everything up.

[pshaughn]

It also clears up #24997 and #24998! It is not solving #24993 AFAICT.