Web Animations CSS: Add unsigned short to support outline-width

Add unsigned short to animatable factory to support outline-width. The
test animations/interpolation/outline-width.html no longer crashes when
run in the virtual/web-animations-css test suite and no longer overflows
to negative values (correctly clamps to unsigned short) so update test
expectations for that.

Change in behavior here to match Firefox's implementation and align with
the CSS spec (outline-width should not be negative).

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23548021

git-svn-id: svn://svn.chromium.org/blink/trunk@157444 bbb929c8-8fbe-4397-9dbb-9b2b20218538