TestScreen shouldn't be marked aura export

ui/aura/test/aura_test_base.cc

Index: ui/aura/test/aura_test_base.cc
diff --git a/ui/aura/test/aura_test_base.cc b/ui/aura/test/aura_test_base.cc
index a7399a8e72901b4116755448bc445bb1f238ecea..203e5336606c96ff4e347432221677c1bd1fbad4 100644
--- a/ui/aura/test/aura_test_base.cc
+++ b/ui/aura/test/aura_test_base.cc
@@ -5,6 +5,7 @@
 #include "ui/aura/test/aura_test_base.h"
 
 #include "ui/aura/root_window.h"
+#include "ui/aura/test/test_screen.h"
 #include "ui/aura/test/test_stacking_client.h"
 
 namespace aura {
@@ -19,6 +20,7 @@ AuraTestBase::~AuraTestBase() {
 void AuraTestBase::SetUp() {
   testing::Test::SetUp();
   root_window_.reset(new aura::RootWindow);
+  gfx::Screen::SetInstance(new aura::TestScreen(root_window_.get()));
   helper_.InitRootWindow(root_window());
   helper_.SetUp();
   stacking_client_.reset(new TestStackingClient(root_window()));