Modify UpdateLibrary to use async RequestUpdateCheck call

chrome/browser/chromeos/login/update_screen.h

Index: chrome/browser/chromeos/login/update_screen.h
diff --git a/chrome/browser/chromeos/login/update_screen.h b/chrome/browser/chromeos/login/update_screen.h
index 2fd986495ef33ee7e36220259878a0af8a7880bb..c1afb0d9409b71f21ebd7121dc863c41861d8d99 100644
--- a/chrome/browser/chromeos/login/update_screen.h
+++ b/chrome/browser/chromeos/login/update_screen.h
@@ -60,6 +60,9 @@ class UpdateScreen: public DefaultViewScreen<chromeos::UpdateView>,
   // Default value is false.
   void SetAllUpdatesCritical(bool is_critical);
 
+  // Returns true if this instance is still active (i.e. has not been deleted).
+  static bool HasInstance(UpdateScreen* inst);
+
  private:
   // Timer notification handlers.
   void OnWaitForRebootTimeElapsed();
@@ -71,6 +74,10 @@ class UpdateScreen: public DefaultViewScreen<chromeos::UpdateView>,
   // If reboot didn't happen - ask user to reboot manually.
   base::OneShotTimer<UpdateScreen> reboot_timer_;
 
+  // Returns a static InstanceSet.
+  typedef std::set<UpdateScreen*> InstanceSet;
+  static InstanceSet& GetInstanceSet();
+
   // True if in the process of checking for update.
   bool checking_for_update_;