Fix compile error on clang

ash/desktop_background/desktop_background_widget_controller.cc

Index: ash/desktop_background/desktop_background_widget_controller.cc
diff --git a/ash/desktop_background/desktop_background_widget_controller.cc b/ash/desktop_background/desktop_background_widget_controller.cc
index 5d2bf71038f005176a0b13cb4f07ff806eb6658b..cb7a369f09120c7e68b654f28ca82e0da4610a65 100644
--- a/ash/desktop_background/desktop_background_widget_controller.cc
+++ b/ash/desktop_background/desktop_background_widget_controller.cc
@@ -59,6 +59,9 @@ ComponentWrapper::ComponentWrapper(
   component_.reset(component);
 }
 
+ComponentWrapper::~ComponentWrapper() {
+}
+
 DesktopBackgroundWidgetController* ComponentWrapper::GetComponent(
     bool pass_ownership) {
   if (pass_ownership)