Web Animations API: Add runtime flag for Web Animations API
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@162601 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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