[DL]: Implement updateAndCommit().

This patch adds updateAndCommit() which updates the subtree cooperatively
and then commits.

R=chrishtr@chromium.org

Bug: 882663
Change-Id: I14b906ff1ccf2ed57df082bfe883bb5d4c6e9ac3
Reviewed-on: https://chromium-review.googlesource.com/c/1422867
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626242}
19 files changed