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

chrome/browser/cocoa/constrained_window_mac.h

Index: chrome/browser/cocoa/constrained_window_mac.h
diff --git a/chrome/browser/cocoa/constrained_window_mac.h b/chrome/browser/cocoa/constrained_window_mac.h
index 612b459ede6dc2cd325d6366ceea365752dd0599..0fd315d1176170facd8ab038d8645e7fd44f46ac 100644
--- a/chrome/browser/cocoa/constrained_window_mac.h
+++ b/chrome/browser/cocoa/constrained_window_mac.h
@@ -117,6 +117,7 @@ class ConstrainedWindowMac : public ConstrainedWindow {
   virtual ~ConstrainedWindowMac();
 
   // Overridden from ConstrainedWindow:
+  virtual void ShowConstrainedWindow();
   virtual void CloseConstrainedWindow();
 
   // Returns the TabContents that constrains this Constrained Window.
@@ -143,6 +144,9 @@ class ConstrainedWindowMac : public ConstrainedWindow {
   // Controller of the window that contains this sheet.
   BrowserWindowController* controller_;
 
+  // Stores if |ShowConstrainedWindow()| was called.
+  bool should_be_visible_;
+
   DISALLOW_COPY_AND_ASSIGN(ConstrainedWindowMac);
 };