Added usage of ScreenContext in EulaScreen.

components/login/screens/screen_context.h

Index: components/login/screens/screen_context.h
diff --git a/components/login/screens/screen_context.h b/components/login/screens/screen_context.h
index 6de612fbe1bd0b3e8d2c1d3ba029983f0d7055e2..bfbba12551b9cb363469886240ea33354af78876 100644
--- a/components/login/screens/screen_context.h
+++ b/components/login/screens/screen_context.h
@@ -9,9 +9,13 @@
 #include <vector>
 
 #include "base/basictypes.h"
+#include "base/callback.h"
+#include "base/callback_list.h"
+#include "base/containers/hash_tables.h"
 #include "base/containers/hash_tables.h"
 #include "base/logging.h"
 #include "base/memory/linked_ptr.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/values.h"
@@ -37,6 +41,10 @@ class LOGIN_EXPORT ScreenContext : public base::NonThreadSafe {
  public:
   typedef std::string KeyType;
   typedef base::Value ValueType;
+  typedef base::Callback<void(const KeyType& key, const base::Value& value)>
+      KeyObserver;
+  typedef base::CallbackList<KeyObserver::RunType> KeyObserversList;
+  typedef KeyObserversList::Subscription KeyObserverSubscription;
 
   ScreenContext();
   ~ScreenContext();
@@ -68,6 +76,10 @@ class LOGIN_EXPORT ScreenContext : public base::NonThreadSafe {
   String16List GetString16List(const KeyType& key,
                                const String16List& default_value) const;
 
+  scoped_ptr<KeyObserverSubscription> AddKeyObserver(
+      const KeyType& key,
+      const KeyObserver& observer);
+
   // Returns true if context has |key|.
   bool HasKey(const KeyType& key) const;
 
@@ -119,6 +131,8 @@ class LOGIN_EXPORT ScreenContext : public base::NonThreadSafe {
   // Contains all pending changes.
   base::DictionaryValue changes_;
 
+  base::hash_map<KeyType, linked_ptr<KeyObserversList>> key_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(ScreenContext);
 };