Don't run Web Animations interpolation tests for prefixed properties

Since we don't support animating prefixed properties in Web Animations,
it doesn't make sense for the interpolation tests to test this. These
tests currently all fail, but they are behaving as expected.

Review URL: https://codereview.chromium.org/680343004

git-svn-id: svn://svn.chromium.org/blink/trunk@184559 bbb929c8-8fbe-4397-9dbb-9b2b20218538