[css-properties-values-api] Don't -webkit-cross-fade <image> values.

It is not spec'ed anywhere that <image> values interpolate in this way.

They should however interpolate as gradients, but we don't have an
InterpolationType for that yet.

R=flackr@chromium.org

Bug: 641877
Change-Id: I1a554d578c505f228f728ad7e09c3d5b4e5117e7
Reviewed-on: https://chromium-review.googlesource.com/1186416
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586267}
2 files changed