[DL] Add lockedElement attribute to DisplayLockContext.

The locked element attribute is useful on the context since it creates
an easy access point to add elements into.

R=chrishtr@chromium.org

Bug: https://github.com/chrishtr/display-locking/issues/30
Change-Id: I726a1db72373bdde261a808b38ccb17424a21348
Reviewed-on: https://chromium-review.googlesource.com/c/1336507
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608792}
3 files changed