Make -webkit-appearance as an alias of -moz-appearance under preference "layout.css.webkit-appearance.enabled" default to false.

Hi mats, could you review the patches for me? The patches are to make -webkit-appearance as an alias of -moz-appearance behind a pref "layout.css.webkit-appearance.enabled" default to false. The stylo part would be reviewed and landed in bug 1429713.

Comment on attachment 8942078 [details] Bug 1429700 Part 1 - Add a pref "layout.css.webkit-appearance.enabled" to control whether we support -webkit-appearance or not. https://reviewboard.mozilla.org/r/212304/#review220310

Comment on attachment 8942079 [details] Bug 1429700 Part 2 - Make -webkit-appearance as an alias of -moz-appearance in gecko. https://reviewboard.mozilla.org/r/212306/#review220312 So when the layout.css.webkit-appearance.enabled pref is enabled, -moz-appearance is *both* an alias and a normal property? This seems weird to me. We can probably solve that by having another pref for the -moz-appearance property that we automatically turn off when webkit-appearance.enabled is enabled and vice versa. But, I think we should probably just skip implementing this in the old style system and implement it only in Stylo. I assume the same issue may arise there though... r- for now because I don't think this is needed.

The Stylo part of this was fixed in bug 1429713, and since the old style system no longer exists this is no longer valid.