Add Source annotation to make build pass with gss turned on.

Change-Id: I1ee0cfef754ca82d3e41de0bed9440d2a0da7de5
diff --git a/user/src/com/google/gwt/user/client/ui/NativeVerticalScrollbar.java b/user/src/com/google/gwt/user/client/ui/NativeVerticalScrollbar.java
index 409d9c1..0cf90dc 100644
--- a/user/src/com/google/gwt/user/client/ui/NativeVerticalScrollbar.java
+++ b/user/src/com/google/gwt/user/client/ui/NativeVerticalScrollbar.java
@@ -89,6 +89,7 @@
   interface UiBinderBundle extends ClientBundle {
     UiBinderBundle INSTANCE = GWT.create(UiBinderBundle.class);
 
+    @Source("com/google/gwt/user/client/ui/NativeVerticalScrollbarUi.css")
     UiStyle nativeVerticalScrollbarUi();
   }