Added usefull utility methods to Screen and ScreenContext.

chrome/browser/chromeos/login/screens/screen_context.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/login/screens/screen_context.h"
 
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace chromeos {
@@ skipping 80 matching lines @@
   DCHECK(CalledOnValidThread());
   DCHECK(diff);
   changes_.Swap(diff);
   changes_.Clear();
 }
 
 void ScreenContext::ApplyChanges(const base::DictionaryValue& diff,
                                  std::vector<std::string>* keys) {
   DCHECK(CalledOnValidThread());
   DCHECK(!HasChanges());
-  DCHECK(keys);
-  keys->clear();
-  keys->reserve(diff.size());
+  if (keys) {
+    keys->clear();
+    keys->reserve(diff.size());
+  }
   base::DictionaryValue::Iterator it(diff);
   while (!it.IsAtEnd()) {
     Set(it.key(), it.value().DeepCopy());
-    keys->push_back(it.key());
+    if (keys)
+      keys->push_back(it.key());
     it.Advance();
   }
   changes_.Clear();
 }
 
 bool ScreenContext::Set(const KeyType& key, base::Value* value) {
   DCHECK(CalledOnValidThread());
   DCHECK(value);
   scoped_ptr<base::Value> new_value(value);
 
   base::Value* current_value;
   bool in_storage = storage_.Get(key, &current_value);
 
   // Don't do anything if |storage_| already contains <|key|, |new_value|> pair.
   if (in_storage && new_value->Equals(current_value))
     return false;
 
   changes_.Set(key, new_value->DeepCopy());
   storage_.Set(key, new_value.release());
   return true;
 }
 
 }  // namespace chromeos