[DL]: Implement lock after append mode.

This patch adds the lock after append mode which "clears" the painted
output while retaining the size information for the locked element.

Also adds a few tests to test the new functionality.

R=chrishtr@chromium.org

Bug: 882663
Change-Id: Id70d7af9d99e919d34691b032db26fec9199e02e
Reviewed-on: https://chromium-review.googlesource.com/c/1401269
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621693}
17 files changed