Add a mojo bridge for PersistentPrefStore.

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 PreferencesService and to ensure that the
 // PreferencesServiceClient is bound at creation time.
 interface PreferencesServiceFactory {
   // Creates a PreferencesService bound to the provided |observer|.
   Create(PreferencesServiceClient observer, PreferencesService& service);
 };
 
 // Used to subscribe to preference changes within PreferenceManager. After
 // requesting to observe, the current values for all requested keys are sent.
 interface PreferencesServiceClient {
   OnPreferencesChanged(mojo.common.mojom.DictionaryValue preferences);
 };
 
 // Manages actual read/write of preference data. Accepts observers who subscribe
 // to preferences, notifying them of changes.
 interface PreferencesService {
   SetPreferences(mojo.common.mojom.DictionaryValue preferences);
   Subscribe(array<string> preferences);
 };
+
+interface PersistentPrefStore {

[tibell, 2017/03/08 03:39:54]

Document.

+  SetValue(string key, mojo.common.mojom.Value? value, uint32 flags);

[tibell, 2017/03/08 03:39:54]

Here you at least want to say that a null |value| deletes.

+
+  CommitPendingWrite();
+  SchedulePendingLossyWrites();
+  ClearMutableValues();
+};
+
+interface PersistentPrefStoreConnector {
+  enum ReadError {
+    NONE = 0,
+    JSON_PARSE = 1,
+    JSON_TYPE = 2,
+    ACCESS_DENIED = 3,
+    FILE_OTHER = 4,
+    FILE_LOCKED = 5,
+    NO_FILE = 6,
+    JSON_REPEAT = 7,
+    // OTHER = 8,  // Deprecated.
+    FILE_NOT_SPECIFIED = 9,
+    ASYNCHRONOUS_TASK_INCOMPLETE = 10,
+  };
+
+  [Sync]
+  Connect() => (
+      ReadError read_error,
+      bool read_only,
+      mojo.common.mojom.DictionaryValue? preferences,

[tibell, 2017/03/08 03:39:54]

I believe null here implies initialization failure? Document that and the
relationship between the returned values.

+      PersistentPrefStore? pref_store);
+};