Pref service: Add initial support for pref registration.

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 0225a8486d4c7a41afd01051589ddf08ac736532..eab0b6074b453d459e102b1a8a5155d9b00a85fc 100644
--- a/services/preferences/public/interfaces/preferences.mojom
+++ b/services/preferences/public/interfaces/preferences.mojom
@@ -112,8 +112,9 @@ interface PrefStoreConnector {
   // prefs in each store and an |observer| interfaces through which updates can
   // be received.
   [Sync]
-  Connect() => (PersistentPrefStoreConnection connection,
-                map<PrefStoreType, PrefStoreConnection> connections);
+  Connect(PrefRegistry pref_registry) =>
+      (PersistentPrefStoreConnection connection,
+       map<PrefStoreType, PrefStoreConnection> connections);
 };
 
 // An interface providing mutation access to a PersistentPrefStore.
@@ -128,6 +129,20 @@ interface PersistentPrefStore {
   ClearMutableValues();
 };
 
+// A registry of all prefs registered by a single client.
+struct PrefRegistry {
+  map<string, PrefRegistration> registrations;
+};
+
+struct PrefRegistration {
+  mojo.common.mojom.Value default_value;
+
+  // A bitfield of flags. Flag values are defined in
+  // PrefRegistry::PrefRegistrationFlags and
+  // PrefRegistrySyncable::PrefRegistrationFlags.
+  uint32 flags;
+};
+
 interface PrefServiceControl {
   // Initializes the pref service. This must be called before the service can
   // be used.