Web Animations API: Add runtime flag for Web Animations API

This flag will be used when we start adding Web Animations' JS API. Work on the API implementation has commenced & this is the first CL.

Review URL: https://codereview.chromium.org/85173002

git-svn-id: svn://svn.chromium.org/blink/trunk@162601 bbb929c8-8fbe-4397-9dbb-9b2b20218538