Add "auto" handling to border-image-width animations

Animation code was not able to handle "auto" values in border-image-width
keyframes. This patch adds it to the NonInterpolableValue along with
appropriate handling for animating between values with matched auto
keywords.

BUG=681424

Review-Url: https://codereview.chromium.org/2696563006
Cr-Commit-Position: refs/heads/master@{#451550}
2 files changed