Web Animations CSS: Animate pseudo elements

This patch hooks up animation of pseudo elements with the Web Animations
engine.

R=esprehn,dstockwell
BUG=258896

Review URL: https://codereview.chromium.org/58853002

git-svn-id: svn://svn.chromium.org/blink/trunk@161427 bbb929c8-8fbe-4397-9dbb-9b2b20218538