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This change uses prefs to store and retrieve app ids for the apps
registered for updates.

Bug: 1020285
Change-Id: I92f08bbc31979fa80e5621e2bd00d1c83cb5c11b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079142Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745199}