Web Animations: Layout tests for responsive interpolation

These layout tests illustrate the intended behavior when we interpolate between specified instead of computed style values.

BUG=361948

Review URL: https://codereview.chromium.org/232593002

git-svn-id: svn://svn.chromium.org/blink/trunk@171746 bbb929c8-8fbe-4397-9dbb-9b2b20218538