We no longer need to composite for clipping because the clip can be
applied in the compositor via property trees.

Change-Id: I740cd8e7b247a7f8bcd98562daedaeef7f1efca3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2235638
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777524}