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..f3e140956b3e4c8788718fc2af0cc2ae3855cad2 100644
--- a/services/preferences/public/interfaces/preferences.mojom
+++ b/services/preferences/public/interfaces/preferences.mojom
@@ -27,3 +27,34 @@ 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);
+};