Add struct traits for base::Value.

services/preferences/public/cpp/pref_observer_store.h

Index: services/preferences/public/cpp/pref_observer_store.h
diff --git a/services/preferences/public/cpp/pref_observer_store.h b/services/preferences/public/cpp/pref_observer_store.h
index c8af343ba06f23d3ab45883e0aaca068fa9ede55..ea474600b4b41de7d3fea5ba259dc3384a24794a 100644
--- a/services/preferences/public/cpp/pref_observer_store.h
+++ b/services/preferences/public/cpp/pref_observer_store.h
@@ -61,7 +61,8 @@ class PrefObserverStore : public ValueMapPrefStore,
                                    base::Value const & value);
 
   // prefs::mojom::PreferenceObserver:
-  void OnPreferencesChanged(const base::DictionaryValue& preferences) override;
+  void OnPreferencesChanged(
+      std::unique_ptr<base::DictionaryValue> preferences) override;
 
   mojo::Binding<prefs::mojom::PreferencesObserver> prefs_binding_;
   prefs::mojom::PreferencesManagerPtr prefs_manager_ptr_;