Web Animations: Promote AnimatableValue type member from variable to virtual function

Simplifies subclassing of AnimatableValue.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/25092008

git-svn-id: svn://svn.chromium.org/blink/trunk@158521 bbb929c8-8fbe-4397-9dbb-9b2b20218538