Revert of Remove some ifdefs from ash since it only supports ChromeOS now.

ash/display/mirror_window_controller.cc

Index: ash/display/mirror_window_controller.cc
diff --git a/ash/display/mirror_window_controller.cc b/ash/display/mirror_window_controller.cc
index 035ab953873c586ab1922eec332106cdfeb0bdca..b0e2d24f58708b56af2c74c3faee2a115031379d 100644
--- a/ash/display/mirror_window_controller.cc
+++ b/ash/display/mirror_window_controller.cc
@@ -200,6 +200,7 @@
       }
 #endif
 
+#if defined(OS_CHROMEOS)
       if (display_manager->IsInUnifiedMode()) {
         host_info->ash_host->ConfineCursorToRootWindow();
         AshWindowTreeHost* unified_ash_host =
@@ -211,6 +212,7 @@
         aura::client::SetScreenPositionClient(host->window(),
                                               screen_position_client_.get());
       }
+#endif
 
       aura::client::SetCaptureClient(host->window(), new NoneCaptureClient());
       host->Show();