Sync: Expose GetRegistered/ForcedDataTypes in SyncService

Previously, GetRegisteredDataTypes and GetForcedDataTypes were only
exposed from the (concrete) ProfileSyncService, not from the (interface)
SyncService, as opposed to GetPreferredDataTypes and GetActiveDataTypes.
It seems right and sensible to expose the registered and forced data
types too.
Concretely, this will allow us to write more code (e.g. PeopleHandler)
in terms of SyncService rather than ProfileSyncService.

Bug: 884159, 910518
Change-Id: I1d9176d4d2ced1dc5defda1b85b6ced2fa51afba
Reviewed-on: https://chromium-review.googlesource.com/c/1365594
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614676}
7 files changed