chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/display/window_tree_host_manager_unittest.cc

Index: ash/display/window_tree_host_manager_unittest.cc
diff --git a/ash/display/window_tree_host_manager_unittest.cc b/ash/display/window_tree_host_manager_unittest.cc
index 99d37cfea15aaf45b8f36e8639d64a0be7b2d74e..2f1a3bc9095ee845d140da31491e57a07eb8eecf 100644
--- a/ash/display/window_tree_host_manager_unittest.cc
+++ b/ash/display/window_tree_host_manager_unittest.cc
@@ -185,11 +185,6 @@ display::Display GetPrimaryDisplay() {
       Shell::GetAllRootWindows()[0]);
 }
 
-display::Display GetSecondaryDisplay() {
-  return display::Screen::GetScreen()->GetDisplayNearestWindow(
-      Shell::GetAllRootWindows()[1]);
-}
-
 class TestHelper {
  public:
   explicit TestHelper(test::AshTestBase* delegate);