[Mac] Avoid "adding unknown subview" warning.

chrome/browser/ui/cocoa/tabs/tab_window_controller.h

Index: chrome/browser/ui/cocoa/tabs/tab_window_controller.h
diff --git a/chrome/browser/ui/cocoa/tabs/tab_window_controller.h b/chrome/browser/ui/cocoa/tabs/tab_window_controller.h
index 407c794978e0bbb6adc82e0487dfe08bab1f8b2d..e9904d90f11ff9d94b16f4da304e6fdefe0ea552 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_window_controller.h
+++ b/chrome/browser/ui/cocoa/tabs/tab_window_controller.h
@@ -18,6 +18,7 @@
 
 @class FastResizeView;
 @class FocusTracker;
+@class NSVisualEffectView;
 @class TabStripView;
 @class TabView;
 
@@ -27,6 +28,9 @@
   base::scoped_nsobject<FastResizeView> tabContentArea_;
   base::scoped_nsobject<NSView> tabStripBackgroundView_;
 
+  // Used to blur the titlebar. nil if window does not have titlebar.
+  base::scoped_nsobject<NSVisualEffectView> visualEffectView_;
+
   // The tab strip overlaps the titlebar of the window.
   base::scoped_nsobject<TabStripView> tabStripView_;