Bug 650372 - Use the style-system format hint directly in gfx, instead of...

Bug 650372 - Use the style-system format hint directly in gfx, instead of mapping to a separate internal enum. r=gfx-reviewers,aosmond,lsalzman

Now that the style system has keywords for this, we don't need to define them in gfx
but can just use the enum directly. (No functional change, just code simplification.)

Differential Revision: https://phabricator.services.mozilla.com/D154238