chromeos-dbus-bindings: Ignore multi-return methods Multiple return arguments are legal in D-Bus. Instead of throwing a fatal error, ignore methods of this sort in the adaptor generator, since libchromeos currently cannot automatically generate returns for such methods. While here, provide a protected getter to the DBus interface pointer, so subclasses can register their own methods. BUG=chromium:419271 TEST=Modified unit tests Change-Id: Idd4ed43ee4cb8fa4249d666d85cadcb465c39cc1 Reviewed-on: https://chromium-review.googlesource.com/220750 Reviewed-by: Alex Deymo <[email protected]> Reviewed-by: Alex Vakulenko <[email protected]> Commit-Queue: Paul Stewart <[email protected]> Tested-by: Paul Stewart <[email protected]>