Make aura_demo work on Mac

ui/gfx/screen_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/screen.h"
 
 #import <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 
 #include <map>
@@ skipping 136 matching lines @@
     }
 
     if (!displays.size())
       return std::vector<gfx::Display>(1, GetPrimaryDisplay());
 
     return displays;
   }
 
   virtual gfx::Display GetDisplayNearestWindow(
       gfx::NativeView view) const OVERRIDE {
-    NSWindow* window = [view window];
+    NSWindow* window = nil;
+#if !defined(USE_AURA)
+    window = [view window];
+#endif
     if (!window)
       return GetPrimaryDisplay();
     NSScreen* match_screen = [window screen];
     if (!match_screen)
       return GetPrimaryDisplay();
     return GetDisplayForScreen(match_screen);
   }
 
   virtual gfx::Display GetDisplayNearestPoint(
       const gfx::Point& point) const OVERRIDE {
@@ skipping 34 matching lines @@
   }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ScreenMac);
 };
 
 }  // namespace
 
 namespace gfx {
 
+#if !defined(USE_AURA)
 Screen* CreateNativeScreen() {
   return new ScreenMac;
 }
+#endif
 
 }