[css-geometry] Please update to Web IDL changes to optional dictionary defaulting

[bzbarsky]

Web IDL recently changed to require explicitly listing a default value for defaulted dictionaries instead of it just magically happening.

In practice what that means is that APIs like this:

 [NewObject] static DOMPointReadOnly fromPoint(optional DOMPointInit other);

need to be written like this instead:

 [NewObject] static DOMPointReadOnly fromPoint(optional DOMPointInit other = {});

(with the addition of = {} on the dictionary argument). This affects a bunch of the geometry APIs.

See whatwg/webidl#750 for details.