[AF] Add chrome://flags entry for enabling USS for wallet data

Bug: 853688
Change-Id: I4069581ed4eea46c04351593a595102783bac438
Reviewed-on: https://chromium-review.googlesource.com/1169479
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582237}
4 files changed