Fixes crash in unittests. This happened as the result of making the

ui/base/native_theme/native_theme_win.cc

Index: ui/base/native_theme/native_theme_win.cc
diff --git a/ui/base/native_theme/native_theme_win.cc b/ui/base/native_theme/native_theme_win.cc
index 36ead677f12fcef4341ffbf396c0a0eae63475c0..1933b45bedf02c8cd022ccc7edb972a16bbc8ff0 100644
--- a/ui/base/native_theme/native_theme_win.cc
+++ b/ui/base/native_theme/native_theme_win.cc
@@ -192,10 +192,15 @@ bool NativeThemeWin::IsClassicTheme(ThemeName name) const {
   return !GetThemeHandle(name);
 }
 
+// TODO(sky): seems like we should default to NativeThemeWin, but that currently
+// breaks a couple of tests (FocusTraversalTest.NormalTraversal in
+// views_unittests).
+#if !defined(USE_AURA)
 // static
 NativeTheme* NativeTheme::instance() {
   return NativeThemeWin::instance();
 }
+#endif
 
 // static
 NativeThemeWin* NativeThemeWin::instance() {