Remove stl_util's STLDeleteContainerPairSecondPointers from prefs.

components/prefs/pref_notifier_impl_unittest.cc

Index: components/prefs/pref_notifier_impl_unittest.cc
diff --git a/components/prefs/pref_notifier_impl_unittest.cc b/components/prefs/pref_notifier_impl_unittest.cc
index a3f12349b9f81df94b9b71a32ee9647d820b066f..517a403936aaed88da382d4d1ec0a561f978dc80 100644
--- a/components/prefs/pref_notifier_impl_unittest.cc
+++ b/components/prefs/pref_notifier_impl_unittest.cc
@@ -59,7 +59,7 @@ class MockPrefNotifier : public PrefNotifierImpl {
     if (observer_iterator == pref_observers()->end())
       return false;
 
-    PrefObserverList* observer_list = observer_iterator->second;
+    PrefObserverList* observer_list = observer_iterator->second.get();
     PrefObserverList::Iterator it(observer_list);
     PrefObserver* existing_obs;
     size_t count = 0;