Add PerformanceObserver.supportedEntryTypes

[pshaughn]

https://w3c.github.io/performance-timeline/#supportedentrytypes-attribute Semantics of what it returns are simple, but it needs to be [SameObject] static readonly attribute FrozenArray<DOMString> which is a lot of adjectives.

[pshaughn]

FrozenArray (#10072) is the one part of this we don't seem to have; some other code is returning "any" as a workaround.

[jdm]

This is the other half of the workaround:

servo/components/script/dom/bindings/utils.rs

Lines 126 to 134 in dbba9ea

	/// Returns a JSVal representing a frozen array of ports
	pub fn to_frozen_array<T: ToJSValConvertible>(convertibles: &[T], cx: SafeJSContext) -> JSVal {
	rooted!(in(*cx) let mut ports = UndefinedValue());
	unsafe { convertibles.to_jsval(*cx, ports.handle_mut()) };
	rooted!(in(*cx) let obj = ports.to_object());
	unsafe { JS_FreezeObject(*cx, RawHandleObject::from(obj.handle())) };
	*ports
	}