Incorrect Document can be used when creating elements during parsing

[cbrewster]

Step 1 of https://html.spec.whatwg.org/multipage/#create-an-element-for-the-token states that the intended parent's node document should be used when creating a new element during parsing.

We currently use the document that is associated with the parser. (line 795)

servo/components/script/dom/servoparser/mod.rs

Lines 787 to 804 in 96b4e06

	fn create_element(&mut self, name: QualName, attrs: Vec<Attribute>, _flags: ElementFlags)
	-> JS<Node> {
	let is = attrs.iter()
	.find(|attr| attr.name.local.eq_str_ignore_ascii_case("is"))
	.map(|attr| LocalName::from(&*attr.value));
	let elem = Element::create(name,
	is,
	&*self.document,
	ElementCreator::ParserCreated(self.current_line),
	CustomElementCreationMode::Synchronous);
	for attr in attrs {
	elem.set_attribute_from_parser(attr.name, DOMString::from(String::from(attr.value)), None);
	}
	JS::from_ref(elem.upcast())
	}

This causes issues when creating an element that is supposed to be inside a template's contents because contents has a different document than the template.

[nox]

Cc @cynicaldevil