[Sync:USS] Implement NigoriStorageImpl

We write NigoriLocalData in encrypted form to corresponding file using
ImportantFileWriter. Reading is just mirrored version of writing.

Bug: 922900
Change-Id: I299f55450e74edac29beaacf7b5dc659f5fd8f8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768424
Commit-Queue: Maksim Moskvitin <[email protected]>
Reviewed-by: Marc Treib <[email protected]>
Cr-Commit-Position: refs/heads/master@{#690014}
4 files changed