Called to execute a command selected from the context menu. Return true (1) if the command was handled or false (0) for the default implementation. See cef_menu_id_t for the command ids that have default implementations. All user-defined command ids should be between MENU_ID_USER_FIRST and MENU_ID_USER_LAST. |params| will have the same values as what was passed to on_before_context_menu(). Do not keep a reference to |params| outside of this callback.