Added usefull utility methods to Screen and ScreenContext.

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

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_SCREEN_CONTEXT_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_SCREEN_CONTEXT_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 53 matching lines @@
   // Returns true if there was changes since last call to
   // GetChangesAndReset().
   bool HasChanges() const;
 
   // Stores all changes since the last call to the
   // GetChangesAndReset() in |diff|.  All previous contents of |diff|
   // will be thrown away.
   void GetChangesAndReset(base::DictionaryValue* diff);
 
   // Applies changes from |diff| to the context. All keys from |diff|
-  // are stored in |keys|.
+  // are stored in |keys|. |keys| argument is optional and can be NULL.
   void ApplyChanges(const base::DictionaryValue& diff,
                     std::vector<std::string>* keys);
 
  private:
   bool Set(const KeyType& key, base::Value* value);
 
   template<typename T>
   T Get(const KeyType& key) {
     DCHECK(CalledOnValidThread());
     const base::Value* value;
@@ skipping 20 matching lines @@
 
   // Contains all pending changes.
   base::DictionaryValue changes_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenContext);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_SCREEN_CONTEXT_H_