libchromeos: dbus: support additional types for variant deserialization

This is needed for supporting cellular modem proxies in shill.

While there, use std::tuple instead of std::pair for deserializing struct
variant type.

BUG=chromium:517680
TEST=build libchromeos
TEST=Run network_3GSmokeTest on device with shill using libchromeos dbus

Change-Id: I6c138ee4a52c2655143929c70d6958d6aff3b05f
1 file changed