[cssom] Allow passing a callback to window.getComputedStyle(…)

[ExE-Boss]

https://drafts.csswg.org/cssom/#dom-window-getcomputedstyle

Right now, window.getComputedStyle(…) is always executed synchronously.

By allowing the last parameter to be a callback, this function could be made asynchronous in a backwards compatible manner.

interface Window {
    getComputedStyle(elt: Element, pseudoElt?: string | null): CSSStyleDeclaration;
    getComputedStyle(elt: Element, callback: (result: CSSStyleDeclaration) => void): void;
    getComputedStyle(elt: Element, pseudoElt: string | null, callback: (result: CSSStyleDeclaration) => void): void;
}

The following assertion holds true for legacy browsers:

// actual will only be undefined in modern browsers.
let actual = window.getComputedStyle(elt, function() {});
// expected will always be a CSSStyleDeclaration
let expected = window.getComputedStyle(elt);

// This works in legacy browsers:
assert.deepEqual(actual, expected);

[upsuper]

If we want it to be asynchronous, it would probably be better to add another function which returns a Promise<CSSStyleDeclaration> or so. But before that, could you elaborate why you want an asynchronous gCS? What usecase are you trying to solve?

[emilio]

getComputedStyle is already "asynchronous" in the sense that it does no work until you call getPropertyValue