[css-masking-1] The syntax of mask shorthand should use <coord-box> instead of <geometry-box>

[BorisChiou]

Per current ED spec, the syntax of <mask-layer> of mask shorthand property still uses <geometry-box>:

<mask-layer> =
 <mask-reference> ||
 <position> [ / <bg-size> ]? ||
 <repeat-style> ||
 <geometry-box> ||
 [ <geometry-box> | no-clip ] ||
 <compositing-operator> ||
 <masking-mode>

However, it's not correct because both mask-origin and mask-clip use <coord-box>, which doesn't include <margin-box>, for example. We probably forgot to update the syntax of mask shorthand when migrating <coord-box> into mask-origin and mask-clip.

[BorisChiou]

I uploaded a PR for this: #552.

If this is an obvious spec issue and we agree with this, we probably can just merge this PR.