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This CL includes:
- Add a util class which sets the preference on the main thread.
- Use utility class in LoginDatabase to set the date on successul deletion of undecryptable passwords.
- Expand DB tests to make sure that the pref is stored.

Bug: 791541
Change-Id: I32c2afc70c6091175cae09a7120de7622b568448
Reviewed-on: https://chromium-review.googlesource.com/1178052
Commit-Queue: Tonko Sabolčec <tsabolcec@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584060}