CSS Animations: Adjust z-index when animating perspective

Styles generated by animations and transitions are currently applied
following adjustment (to avoid triggering transitions on adjustment).

Special handling existed for mirroring the adjustment behavior of
auto z-index when animating transform, however this failed to account
for animations of perspective. This has been broken as far back as
I am able to bisect and remains broken in WebKit for the same reason.

Review URL: https://codereview.chromium.org/219013002

git-svn-id: svn://svn.chromium.org/blink/trunk@170498 bbb929c8-8fbe-4397-9dbb-9b2b20218538