MessageEvent and ExtendableMessageEvent have some issues

[Manishearth]

The implementations of MessageEvent and ExtendableMessageEvent have some issues.

Firstly, they store Vec<DomRoot<T>> in on their DOM object, they should be using Vec<Dom<T>>:

servo/components/script/dom/messageevent.rs

Line 63 in a77318b

ports: DomRefCell<Vec<DomRoot<MessagePort>>>,

servo/components/script/dom/extendablemessageevent.rs

Line 34 in a77318b

ports: Vec<DomRoot<MessagePort>>,

Secondly, they're constructing the type directly in new() instead of using a new_inherited

servo/components/script/dom/extendablemessageevent.rs

Lines 48 to 54 in a77318b

	let ev = Box::new(ExtendableMessageEvent {
	event: ExtendableEvent::new_inherited(),
	data: Heap::default(),
	origin,
	lastEventId,
	ports,
	});

servo/components/script/dom/messageevent.rs

Lines 86 to 93 in a77318b

	let ev = Box::new(MessageEvent {
	event: Event::new_inherited(),
	data: Heap::default(),
	source: DomRefCell::new(source.map(|source| source.into())),
	origin: DomRefCell::new(origin),
	lastEventId: DomRefCell::new(lastEventId),
	ports: DomRefCell::new(ports),
	});

We should fix these issues.

cc @gterzian