Enable InputDeviceServer/InputDeviceClient in mash.

ash/sysui/sysui_application.h

Index: ash/sysui/sysui_application.h
diff --git a/ash/sysui/sysui_application.h b/ash/sysui/sysui_application.h
index 0b32a167abddb5f106a65729f1fbf11dddd97555..b046ba5dd17ebbfddeab92653f998ba57dd299b5 100644
--- a/ash/sysui/sysui_application.h
+++ b/ash/sysui/sysui_application.h
@@ -9,6 +9,7 @@
 
 #include "ash/sysui/public/interfaces/wallpaper.mojom.h"
 #include "base/macros.h"
+#include "components/mus/public/cpp/input_devices/input_device_client.h"
 #include "mash/shelf/public/interfaces/shelf.mojom.h"
 #include "mojo/public/cpp/bindings/binding_set.h"
 #include "services/shell/public/cpp/shell_client.h"
@@ -49,6 +50,9 @@ class SysUIApplication
       shelf_controller_bindings_;
   mojo::BindingSet<mojom::WallpaperController> wallpaper_controller_bindings_;
 
+  // Subscribes to updates about input-devices.
+  input_device::InputDeviceClient input_device_client_;
+
   DISALLOW_COPY_AND_ASSIGN(SysUIApplication);
 };