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This historic mechanism to inject dependencies into ProfileSyncService
doesn't belong in SyncClient (dependencies to core sync logic / engine
itself), and it's questionable whether it should remain in this form in
BrowserSyncClient, where feature-specific dependencies are injected, and
where multiple similar approaches coexist.

Arguably, the most elegant and universal way to inject a dependency
to a sync controller is to have full access to the KeyedService that
owns the local model. Most controllers are simple and don't actually
need this: all they need is a SyncableService or
ModelTypeControllerDelegate. However, that introduces code
inconsistencies that can be confusing.

In this patch, the approach is unified for the remaining
SyncableService types, after migrating history delete directives and
preferences). The corresponding keyed services are directly injected via
BrowserSyncClient and hence SyncClient::GetSyncableServiceForType() can
be deleted.

Change-Id: I8e80482e8657624dc7c1e15b6bcf0409ae4d1336
Bug: 915154
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463617Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816550}