[AF wallet] Implement storage keys and client tags for wallet data.

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

Bug: 853688
Change-Id: If86916ef6486b104685e615c68010ac9da281b61
Reviewed-on: https://chromium-review.googlesource.com/1148962
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578302}
4 files changed