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* Workaround for crbug.com/1139806.
* Writes the CCT scan completion time into a Chrome pref in case
  Chrome is open when the scan completes.
* Uses this timestamp alongside the one from the registry in the
  Chrome cleaner child of safety check.

Bug: 1087263,1139806
Change-Id: Ia3fda6e87058ba76089ce22e8367f41d8e0b8512
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484700Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820895}