[Sync::USS] Introduce about flag for USS passwords sync

This allows more easy testing of the feature under development/
experimentation.

Bug: 902349
Change-Id: I6c8c65c7c9a28ebdb0e40ed5d3a821981ad27ebb
Reviewed-on: https://chromium-review.googlesource.com/c/1454616
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629655}
5 files changed