Factor out logic from SessionsSyncManager to SyncedSessionTracker

The change makes SessionsSyncManager smaller which is good for our
upcoming changes to introduce an alternative implementation based on
USS.

Bug: 681921
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I6e8ed8b3b588b6a8da5e9b6b90be6ed9f479b04c
Reviewed-on: https://chromium-review.googlesource.com/918863
Commit-Queue: Mikel Astiz <[email protected]>
Reviewed-by: Nicolas Zea <[email protected]>
Cr-Commit-Position: refs/heads/master@{#539444}
5 files changed