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When unique db is missing and migration state says it should, the
database could have been deleted by other means. In this case just
return an empty shared db instead of error.

Change-Id: I5b15955662405b306583ed1942d8591703ebf942
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2130685Reviewed-by: Salvador Guerrero <salg@google.com>
Commit-Queue: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756947}