[System Web Apps] Update System Web Apps only on Chrome version change.

This CL causes System Web Apps to only update on Chrome version change
rather than at every startup. This behavior will only apply in official
builds so that developers are still getting constant System Web App
refreshes.

Bug: 836128
Change-Id: I4a6c19be734305fa42aa03c304992c862b7306e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1560872
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651178}
9 files changed