Web Animations CSS: Fix interpolation of clip between rect and auto

Note that the existing implementation fails clip-interpolation as the
underlying value for clip is only set to auto.

Interpolation test helper was updated to normalize -0 to 0.

BUG=279684

Review URL: https://codereview.chromium.org/26775006

git-svn-id: svn://svn.chromium.org/blink/trunk@159563 bbb929c8-8fbe-4397-9dbb-9b2b20218538