Port background-{color, position, size}-interpolation.html to WPT

Bug: 900581
Change-Id: Ic4e14f4add81f40add7d80906ca402cdd1ed6334
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1756984
Reviewed-by: Xida Chen <[email protected]>
Commit-Queue: Stephen McGruer <[email protected]>
Cr-Commit-Position: refs/heads/master@{#688099}
8 files changed