Introduce PrefRegistrySyncable, simplifying PrefServiceSyncable.

chrome/test/base/testing_pref_service.h

Index: chrome/test/base/testing_pref_service.h
diff --git a/chrome/test/base/testing_pref_service.h b/chrome/test/base/testing_pref_service.h
index 9330cfb7bce6d347b1b859a78eb8cab8ff8e24ca..f5d9407259af5c6974795e571688bb9f5aab3c7a 100644
--- a/chrome/test/base/testing_pref_service.h
+++ b/chrome/test/base/testing_pref_service.h
@@ -14,6 +14,7 @@
 class PrefModelAssociator;
 class PrefNotifierImpl;
 class PrefRegistrySimple;
+class PrefRegistrySyncable;
 class TestingBrowserProcess;
 class TestingPrefStore;
 
@@ -101,6 +102,12 @@ class TestingPrefServiceSyncable
   TestingPrefServiceSyncable();
   virtual ~TestingPrefServiceSyncable();
 
+  // This is provided as a convenience; on a production PrefService
+  // you would do all registrations before constructing it, passing it
+  // a PrefRegistry via its constructor (or via
+  // e.g. PrefServiceBuilder).
+  PrefRegistrySyncable* registry();
+
  private:
   DISALLOW_COPY_AND_ASSIGN(TestingPrefServiceSyncable);
 };