Web Animations CSS: Templatize the integer round clamp routines

Use a template to implement the animatiableValueTo<some int type>
routines, and use the template.  Note: I'll remove old routines
in a followup patch (patch diff is easier to read that way).

No behavioral change, with existing tests playing coverage cop.

Review URL: https://chromiumcodereview.appspot.com/23967009

git-svn-id: svn://svn.chromium.org/blink/trunk@157590 bbb929c8-8fbe-4397-9dbb-9b2b20218538