Both can be used to optionally initialize an object, but with
base::Optional it will be stack-allocated.

R=ahaas@chromium.org

Change-Id: I9977e1b2e0532505f8582cc68e27687aaeebd33d
Reviewed-on: https://chromium-review.googlesource.com/781920Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49548}