Introduce skeleton code for Wallet USS implementation

No actual implementation included, just the basic skeleton files and
classes, the required plumbing and the associated feature toggle.

Follow-up patches will introduce the actual implementation, to
ultimately replace AutofillWalletSyncableService.

Bug: 853688
Change-Id: Ia0963fbad2d3e046686e93aea6468283bad186fd
Reviewed-on: https://chromium-review.googlesource.com/1147223
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577570}
9 files changed