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
Revert "Remove ShouldSetAttached flag to lazyAttach"

This reverts commit dfc6e91da01f3573f34d386c1c7db40838db7296.

git-svn-id: svn://svn.chromium.org/blink/trunk@157445 bbb929c8-8fbe-4397-9dbb-9b2b20218538