merge_pr_3

Port z-index interpolation to wpt

Firefox passed
Chrome failed. Bug filed: https://crbug.com/375982

Bug: 900581
Change-Id: I33488661e2938c3fae6bef27ea6672b77047de84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925347
Reviewed-by: Xida Chen xidachen@chromium.org
Commit-Queue: Hao Sheng haozhes@chromium.org
Cr-Commit-Position: refs/heads/master@{#718319}