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

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);

Read the CSSOM getComputedStyle section at the linked specification URL and compare it with the proposed overloads in the issue. Determine the standards behavior and compatibility requirements for the callback form. Done means the Working Draft reflects an agreed callback API and its legacy-browser behavior.