Don't apply (-webkit-)mask-composite on the bottom mask layer

 "If there is no further mask layer, the compositing operator must
  be ignored."

  (https://drafts.fxtf.org/css-masking/#the-mask-composite)

Bug: 915847
Change-Id: Ia42f188160cf84648e364829c7733dbb0895936b
Reviewed-on: https://chromium-review.googlesource.com/c/1384251
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#617865}
3 files changed