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

chrome/browser/tab_contents/constrained_window.h

Index: chrome/browser/tab_contents/constrained_window.h
diff --git a/chrome/browser/tab_contents/constrained_window.h b/chrome/browser/tab_contents/constrained_window.h
index 057ee2745f56e3b1caa5984f21857166dcc0b58b..dacf8c7363a4e1248f61802c2b84ba4d6d2e8d80 100644
--- a/chrome/browser/tab_contents/constrained_window.h
+++ b/chrome/browser/tab_contents/constrained_window.h
@@ -35,10 +35,15 @@ class ConstrainedWindow {
   // 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;
 };