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Before this change, a missing entry of |overlay_image| would reset it. Now
to unset it one needs to add an invalid (e.g. empty) setting for it.

Bug: b/161870830
Change-Id: I9a7fa1b11fe437071a7e4bbea11b44fb95f3b1c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315858Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Sandro Maggi <sandromaggi@google.com>
Cr-Commit-Position: refs/heads/master@{#791192}