Remove support of the -webkit-text value from {-webkit-,}background-clip

Usage is essentially/rounding to zero [1].

[1] https://www.chromestatus.com/metrics/feature/timeline/popularity/941

BUG=586485

Review URL: https://codereview.chromium.org/1708173002

Cr-Commit-Position: refs/heads/master@{#376421}
5 files changed