TestScreen shouldn't be marked aura export

ui/gfx/screen_ash.cc

Index: ui/gfx/screen_ash.cc
diff --git a/ui/gfx/screen_aura.cc b/ui/gfx/screen_ash.cc
similarity index 88%
rename from ui/gfx/screen_aura.cc
rename to ui/gfx/screen_ash.cc
index ea4216fb557846ae6f90271e7155946d80027f78..9df6b84af516391f8eeb03fe276fb3768766f287 100644
--- a/ui/gfx/screen_aura.cc
+++ b/ui/gfx/screen_ash.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -9,9 +9,8 @@
 
 namespace gfx {
 
-// gfx can't depend upon aura, otherwise we have circular dependencies. So,
-// gfx::Screen is pluggable for aura and Desktop plugs in the real
-// implementation.
+// gfx can't depend upon ash, otherwise we have circular dependencies. So,
+// gfx::Screen is pluggable and Desktop plugs in the real implementation.
 
 // static
 Screen* Screen::instance_ = NULL;