Web Animations: Support animating the z-index CSS property

This patch enables the Web Animations implementation to animate the
z-index property.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23587020

git-svn-id: svn://svn.chromium.org/blink/trunk@157661 bbb929c8-8fbe-4397-9dbb-9b2b20218538