The previous approach of encapsulating the pref service from chrome
regressed memory. Remove that encapsulation by moving the source of
truth back into the browser "service" and using those PrefStores
directly from the pref service, bypassing any mojo messages.

Remove the control and registration interfaces which are no longer
needed since the pref service is directly provided with its PrefStores.
Also move the pref service to the UI thread, where the PrefStores
already live.

Other services may still access prefs via the pref service using the
existing sync API (with a local copy of requested prefs), but are
responsible for managing the increased memory footprint that could
result.

Change PersistentPrefStoreImpl to never start a pref read; the embedder
is reponsible for ensuring the underlying PersistentPrefStore is either
already initialised, or is in the process of being initialised. Change
PersistentPrefStoreImpl to always observe the underlying
PersistentPrefStore to detect changes originating from inside the
embedder. Duplicate the update filtering from the per-connection class
to ensure that each client continues to receive each update from other
clients exactly once and never be notified about changes it instigated.

Bug: 654988
Change-Id: I7eb9e427c9f66d1eb4526d6ea8d9ca6e61c87262
Reviewed-on: https://chromium-review.googlesource.com/554653


Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486660}