Add a mojo bridge for PersistentPrefStore.

services/preferences/public/interfaces/preferences.mojom

Index: services/preferences/public/interfaces/preferences.mojom
diff --git a/services/preferences/public/interfaces/preferences.mojom b/services/preferences/public/interfaces/preferences.mojom
index 1978e4b671fc5a85ccbd004317b7e515bffa0d90..94455a1c00040115a37d610a66de7b62e2a619ce 100644
--- a/services/preferences/public/interfaces/preferences.mojom
+++ b/services/preferences/public/interfaces/preferences.mojom
@@ -27,3 +27,35 @@ interface PreferencesService {
   SetPreferences(mojo.common.mojom.DictionaryValue preferences);
   Subscribe(array<string> preferences);
 };
+
+interface PersistentPrefStore {
+  SetValue(string key, mojo.common.mojom.Value value, uint32 flags);
+  RemoveValue(string key, uint32 flags);
+
+  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,
+      PersistentPrefStore? pref_store);
+};