Web Animations: FillMode Auto

Auto is equivalent to Both, except if the timed item is an Animation.
When the timed item is an Animation, Auto is equivalent to None.

See sections 6.4.1 and 6.5.2 of the Web Animations spec.

BUG=

Review URL: https://codereview.chromium.org/152423002

git-svn-id: svn://svn.chromium.org/blink/trunk@166912 bbb929c8-8fbe-4397-9dbb-9b2b20218538