Web Animations: Remove WebAnimations runtime feature in favor of WebAnimationsCSS

The WebAnimations and WebAnimationsCSS flags were not used consistently. As
WebAnimationsCSS is approaching shipping we can block other Web Animations
features behind it.

R=steveblock
TBR=abarth

Review URL: https://codereview.chromium.org/85463002

git-svn-id: svn://svn.chromium.org/blink/trunk@162890 bbb929c8-8fbe-4397-9dbb-9b2b20218538