[Autofill wallet] Implement storage keys and client tags for metadata

This CL implements GetStorageKey() and GetClientTag() functions of the
wallet_metadata USS bridge. The implementation is compatible with the
previous syncable service code.

Bug: 853688
Change-Id: Iafcb631481ccdfa832648a66bb73e1b65b95d524
Reviewed-on: https://chromium-review.googlesource.com/1150155
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578220}
3 files changed