This CL replaces some Handles which can be empty by MaybeHandle. This
documents that they can be empty, and forces a check before using them.

R=ahaas@chromium.org

Change-Id: Iefb9ae76617c45d2304b0a620dc082ab9c7b0585
Reviewed-on: https://chromium-review.googlesource.com/574593Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46733}