Added usefull utility methods to Screen and ScreenContext.

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

Index: chrome/browser/chromeos/login/screens/screen_context.h
diff --git a/chrome/browser/chromeos/login/screens/screen_context.h b/chrome/browser/chromeos/login/screens/screen_context.h
index c0dc4460c8331e4ea853977390c3d4f656221556..4878f89824c835c324e6f5e270166008c4b47eb2 100644
--- a/chrome/browser/chromeos/login/screens/screen_context.h
+++ b/chrome/browser/chromeos/login/screens/screen_context.h
@@ -71,7 +71,7 @@ class ScreenContext : public base::NonThreadSafe {
   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| is optional and can be NULL.

[Denis Kuznetsov (DE-MUC), 2014/06/11 14:29:17]

"|Keys| are" ? 

[dzhioev (left Google), 2014/06/18 14:43:39]

Rephrased to "|keys| argument is optional". 

[Denis Kuznetsov (DE-MUC), 2014/06/19 19:16:36]

Nit : capitalize to "Keys"

[dzhioev (left Google), 2014/06/20 07:24:16]

It is a name of argument, I don't think we should capitalize it.
$ git gs "\. |[a-z]" | wc -l
2298

   void ApplyChanges(const base::DictionaryValue& diff,
                     std::vector<std::string>* keys);