Web Animations CSS: Restart animations from the beginning when display is changed from 'none'

The spec [1] requires that setting the display property of a running animation
to 'none' causes it to be cancelled, and that updating it to any other value
causes the animation to start from the beginning.

Also adds a test to verify this behavior.

[1] http://dev.w3.org/csswg/css-animations/#animations

BUG=257591
R=dstockwell

Review URL: https://codereview.chromium.org/63673002

git-svn-id: svn://svn.chromium.org/blink/trunk@161909 bbb929c8-8fbe-4397-9dbb-9b2b20218538