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This patch uses a schema bump to do a one-time check of all blob files
on disk.  http://crbug.com/1131151 is a bug where under some rare
circumstances blob files may be written and then secretly deleted,
leaving the database with knowledge of an external file that doesn't
exist.

During this one-time check, if any blob files are missing, then the
database is marked as corrupted and will be deleted.  We use a schema
bump to only do this once.

Bug: 1143975
Change-Id: Ibb78e2bfaa4a1ba1becd77b88b5e07b8e0f3fe6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508645
Commit-Queue: enne <enne@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826411}