Add struct traits for base::Value.

services/preferences/public/cpp/pref_observer_store.cc

Index: services/preferences/public/cpp/pref_observer_store.cc
diff --git a/services/preferences/public/cpp/pref_observer_store.cc b/services/preferences/public/cpp/pref_observer_store.cc
index 7bf6f7f2f158c330bae8d220c85a11a06744800e..fac38338cc1e52b0333777d099af287ebae289d9 100644
--- a/services/preferences/public/cpp/pref_observer_store.cc
+++ b/services/preferences/public/cpp/pref_observer_store.cc
@@ -78,14 +78,14 @@ void PrefObserverStore::SetValueOnPreferenceManager(const std::string& key,
   if (keys_.find(key) == keys_.end())
     return;
 
-  base::DictionaryValue prefs;
-  prefs.Set(key, value.CreateDeepCopy());
-  prefs_manager_ptr_->SetPreferences(prefs);
+  auto prefs = base::MakeUnique<base::DictionaryValue>();
+  prefs->Set(key, value.CreateDeepCopy());
+  prefs_manager_ptr_->SetPreferences(std::move(prefs));
 }
 
 void PrefObserverStore::OnPreferencesChanged(
-    const base::DictionaryValue& preferences) {
-  for (base::DictionaryValue::Iterator it(preferences); !it.IsAtEnd();
+    std::unique_ptr<base::DictionaryValue> preferences) {
+  for (base::DictionaryValue::Iterator it(*preferences); !it.IsAtEnd();
        it.Advance()) {
     if (keys_.find(it.key()) == keys_.end())
       continue;