Add support for repeating gradients in the cached fast path.

Documentation:
https://bugzilla.mozilla.org/attachment.cgi?id=9138638

Differential Revision: https://phabricator.services.mozilla.com/D69476

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1b2acec8316617e5ce2972b66e11d0955bfd72e7