merge_pr_20005

Port text-shadow interpolation tests to wpt

Both firefox and chrome failed with the bug filed: https://crbug.com/995349

Bug: 900581
Change-Id: I037f772aa391de65cefc2267390d28f40e0bdde5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891435
Commit-Queue: Hao Sheng haozhes@chromium.org
Reviewed-by: Xida Chen xidachen@chromium.org
Cr-Commit-Position: refs/heads/master@{#711713}