There used to be a method in ContextMenuItem to check if share menu item was supported or not, but since r182573, there's a method to get the share menu item. If the returned menu item is null, it's not added to the menu, but we are not returning a null ContextMenu item even though we don't support share menu item.
Created attachment 251948 [details] Patch
Committed r183547: <http://trac.webkit.org/changeset/183547>