Add DisplayLock idl with a sample layout test.

This patch adds an idl for acquireDisplayLock and DisplayLockContext
with a schedule function. Also adds a naive implementation that just
runs the callback immediately, which makes the layout test pass.

The test should keep passing whenever we actually start implementing
the feature.

See the bug for description of the feature.

R=chrishtr@chromium.org

Bug: 882663
Change-Id: Ia8770e2ca3e6387eec3946c770d0f47cf57d23b4
Reviewed-on: https://chromium-review.googlesource.com/c/1258247
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596927}
16 files changed