Web Animations API: Add runtime flag for minimal element.animate feature

Includes:
* element.animate(sequence<Keyframe> effect, optional (double or TimingInput) timing)
* [NoInterfaceObject] AnimationPlayer with only finish event and cancel() method

BUG=327564

Review URL: https://codereview.chromium.org/238633002

git-svn-id: svn://svn.chromium.org/blink/trunk@171536 bbb929c8-8fbe-4397-9dbb-9b2b20218538