Make aura_demo work on Mac

ui/gfx/screen_mac.mm

Index: ui/gfx/screen_mac.mm
diff --git a/ui/gfx/screen_mac.mm b/ui/gfx/screen_mac.mm
index 51c0b665b7e34274b98cf4c2d3386898af8c98f3..377c8592210b93f00da814c6383c2343d7c28fe7 100644
--- a/ui/gfx/screen_mac.mm
+++ b/ui/gfx/screen_mac.mm
@@ -154,7 +154,10 @@ class ScreenMac : public gfx::Screen {
 
   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];
@@ -209,8 +212,10 @@ class ScreenMac : public gfx::Screen {
 
 namespace gfx {
 
+#if !defined(USE_AURA)
 Screen* CreateNativeScreen() {
   return new ScreenMac;
 }
+#endif
 
 }