[Autofill wallet metadata] Implement GetData() in the USS bridge

This CL implements the functions to load the data of the model type. In
particular, the function GetAllDataForDebugging() unblocks writing
unit-tests for further changes.

Bug: 853688
Change-Id: I6705ed3fd76b83fc59538c2d9e5b4f4a12b57436
Reviewed-on: https://chromium-review.googlesource.com/1151298
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578333}
3 files changed