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

chrome/browser/gtk/constrained_window_gtk.h

Index: chrome/browser/gtk/constrained_window_gtk.h
diff --git a/chrome/browser/gtk/constrained_window_gtk.h b/chrome/browser/gtk/constrained_window_gtk.h
index 91fa9ac54c79f1c5c7bad21252481b40e8ecf68c..e8666654bd3d7da6a7e81f14824418d9864651fc 100644
--- a/chrome/browser/gtk/constrained_window_gtk.h
+++ b/chrome/browser/gtk/constrained_window_gtk.h
@@ -32,6 +32,7 @@ class ConstrainedWindowGtk : public ConstrainedWindow {
   virtual ~ConstrainedWindowGtk();
 
   // Overridden from ConstrainedWindow:
+  virtual void ShowConstrainedWindow();
   virtual void CloseConstrainedWindow();
 
   // Returns the TabContents that constrains this Constrained Window.
@@ -58,6 +59,9 @@ class ConstrainedWindowGtk : public ConstrainedWindow {
   // Delegate that provides the contents of this constrained window.
   ConstrainedWindowGtkDelegate* delegate_;
 
+  // Stores if |ShowConstrainedWindow()| has been called.
+  bool visible_;
+
   DISALLOW_COPY_AND_ASSIGN(ConstrainedWindowGtk);
 };