Switch Preferences to use Factory

services/preferences/public/interfaces/preferences.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module prefs.mojom;
 
 import "mojo/common/values.mojom";
 
 const string kServiceName = "preferences";
 
+// Used for the creation of a PreferencesManager and to ensure that the
+// PreferencesObserver is bound at creation time.
+interface PreferencesFactory {
+  // Creates a PreferencesManager bound to the provided |observer|.
+  Create(PreferencesObserver observer, PreferencesManager& manager);
+};
+
 // Used to subscribe to preference changes within PreferenceManager. After
 // requesting to observe, the current values for all requested keys are sent.
 interface PreferencesObserver {
   OnPreferencesChanged(mojo.common.mojom.DictionaryValue preferences);
 };
 
 // Manages actual read/write of preference data. Accepts observers who subscribe
 // to preferences, notifying them of changes.
 interface PreferencesManager {
-  AddObserver(PreferencesObserver client);
   SetPreferences(mojo.common.mojom.DictionaryValue preferences);
   Subscribe(array<string> preferences);
 };