Mac: Only show one per-tab sheet at a time 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..83331c7e4bd8e3c73913ce5b6e2d1164fa2a07e9 100644
--- a/chrome/browser/cocoa/constrained_window_mac.h
+++ b/chrome/browser/cocoa/constrained_window_mac.h
@@ -125,9 +125,12 @@ class ConstrainedWindowMac : public ConstrainedWindow {
   // Returns the window's delegate.
   ConstrainedWindowMacDelegate* delegate() { return delegate_; }
 
-  // Makes the constrained window visible, if it is not yet visible.
+  // Tells |controller_| that the sheet would like to be displayed.
   void Realize(BrowserWindowController* controller);
 
+  // Called by |controller_| to inform the sheet that it now is visible.
+  void SetVisible();
+
  private:
   friend class ConstrainedWindow;