Web Animations CSS: Add int to support outline-offset

Add int to animatable factory to support outline-offset. The test
animations/interpolation/outline-offset-interpolation.html no longer
crashes when run in the virtual/web-animations-css test suite so
update test expectations for that.

No change in behavior, so no new tests. Covered by existing test
animations/interpolation/outline-offset-interpolation.html

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23842006

git-svn-id: svn://svn.chromium.org/blink/trunk@157570 bbb929c8-8fbe-4397-9dbb-9b2b20218538