Enforce that only one constrained window is displayed at once per tab.

chrome/browser/tab_contents/constrained_window.h

 // Copyright (c) 2006-2008 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_TAB_CONTENTS_CONSTRAINED_WINDOW_H_
 #define CHROME_BROWSER_TAB_CONTENTS_CONSTRAINED_WINDOW_H_
 
 #include "build/build_config.h"
 
 // The different platform specific subclasses use different delegates for their
@@ skipping 17 matching lines @@
 // ConstrainedWindow
 //
 //  This interface represents a window that is constrained to a TabContents'
 //  bounds.
 //
 class ConstrainedWindow {
  public:
   // Create a Constrained Window that contains a platform specific client
   // area. Typical uses include the HTTP Basic Auth prompt. The caller must
   // provide a delegate to describe the content area and to respond to events.
+  // The ConstrainedWindow should not be visible after creation.
   static ConstrainedWindow* CreateConstrainedDialog(
       TabContents* owner,
       ConstrainedWindowDelegate* delegate);
 
+  // Makes the Constrained Window visible. Only one Constrained Window is shown
+  // at a time per tab.
+  virtual void ShowConstrainedWindow() = 0;
+
   // Closes the Constrained Window.
   virtual void CloseConstrainedWindow() = 0;
 };
 
 #endif  // CHROME_BROWSER_TAB_CONTENTS_CONSTRAINED_WINDOW_H_