[Sync:USS] Implement methods in NigoriModelTypeProcessor

This CL implements the following 3 methods:

NigoriModelTypeProcessor::ConnectSync()
NigoriModelTypeProcessor::DisconnectSync()
NigoriModelTypeProcessor::GetLocalChanges()

Bug: 922900
Change-Id: I26da20bb96c4c773a91a5409820667f445ccc262
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1550470
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652096}