Make ui::InputDeviceEventObserver provide a default implementation

ash/virtual_keyboard_controller.cc

Index: ash/virtual_keyboard_controller.cc
diff --git a/ash/virtual_keyboard_controller.cc b/ash/virtual_keyboard_controller.cc
index d043dd32dfe2252fd05b1f50192a3edebad2c6f3..3ecc0bc271c1f3164438eebbd4b1c4ef2de29217 100644
--- a/ash/virtual_keyboard_controller.cc
+++ b/ash/virtual_keyboard_controller.cc
@@ -67,12 +67,6 @@ void VirtualKeyboardController::OnKeyboardDeviceConfigurationChanged() {
   UpdateDevices();
 }
 
-void VirtualKeyboardController::OnMouseDeviceConfigurationChanged() {
-}
-
-void VirtualKeyboardController::OnTouchpadDeviceConfigurationChanged() {
-}
-
 void VirtualKeyboardController::ToggleIgnoreExternalKeyboard() {
   ignore_external_keyboard_ = !ignore_external_keyboard_;
   UpdateKeyboardEnabled();