[SendTabToSelf] add Sync Plumbing for STTS datatype on IOS

Adding objective-c factories and sync clients for SendTabToSelf,
and changing browser_sync code to make use of those new classes.

Bug: 910390
Change-Id: Ieae03174e8c1ff02e2e2de845247f037e805a811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1488118
Commit-Queue: Jeffrey Cohen <jeffreycohen@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643392}
14 files changed