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 fd726018813b86858bc1b44af76768ed4ec09ec0..28856d734226c2bbdc17351236b8affbb3ed927e 100644
--- a/chrome/test/base/testing_pref_service.h
+++ b/chrome/test/base/testing_pref_service.h
@@ -10,10 +10,12 @@
 #include "base/prefs/testing_pref_store.h"
 #include "chrome/browser/prefs/pref_registry.h"
 #include "chrome/browser/prefs/pref_service.h"
+#include "chrome/browser/prefs/pref_service_syncable.h"
 
 class PrefModelAssociator;
 class PrefNotifierImpl;
 class PrefRegistrySimple;
+class PrefRegistrySyncable;
 class TestingBrowserProcess;
 class TestingPrefStore;
 
@@ -102,6 +104,12 @@ 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);
 };