Clean up includes in pref_service_syncable.cc

In particular, it doesn't depend on the mojo preferences service, so
eliminate those includes.

Forward declare a couple things in pref_service_syncable.h as well.

Bug: none
Test: compiles
Change-Id: If7d70388fb5903fb932bd4928a993d1a80cea55e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879568
Commit-Queue: Tim Schumann <tschumann@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Tim Schumann <tschumann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709393}

components/sync_preferences/pref_service_syncable.cc

@@ -8,7 +8,6 @@
 
 #include "base/bind.h"
 #include "base/callback.h"
-#include "base/files/file_path.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/value_conversions.h"
 #include "components/pref_registry/pref_registry_syncable.h"
@@ -17,13 +16,10 @@
 #include "components/prefs/overlay_user_pref_store.h"
 #include "components/prefs/pref_notifier_impl.h"
 #include "components/prefs/pref_registry.h"
+#include "components/prefs/pref_value_store.h"
 #include "components/sync_preferences/pref_model_associator.h"
 #include "components/sync_preferences/pref_service_syncable_observer.h"
-#include "services/preferences/public/cpp/in_process_service_factory.h"
-#include "services/preferences/public/cpp/persistent_pref_store_client.h"
-#include "services/preferences/public/cpp/pref_registry_serializer.h"
-#include "services/preferences/public/mojom/preferences.mojom.h"
-#include "services/service_manager/public/cpp/connector.h"
+#include "components/sync_preferences/synced_pref_observer.h"
 
 #if defined(OS_CHROMEOS)
 #include "chromeos/constants/chromeos_features.h"

components/sync_preferences/pref_service_syncable.h

@@ -13,11 +13,12 @@
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "components/pref_registry/pref_registry_syncable.h"
 #include "components/prefs/pref_service.h"
-#include "components/prefs/pref_value_store.h"
 #include "components/sync_preferences/pref_model_associator.h"
-#include "components/sync_preferences/synced_pref_observer.h"
+
+class PrefValueStore;
 
 namespace syncer {
 class SyncableService;
@@ -27,6 +28,7 @@ namespace sync_preferences {
 
 class PrefModelAssociatorClient;
 class PrefServiceSyncableObserver;
+class SyncedPrefObserver;
 
 // A PrefService that can be synced. Users are forced to declare
 // whether preferences are syncable or not when registering them to