shill: Add bindings for org.freedesktop.ModemManager1.Modem.Location

This CL declares and implements a proxy interface for
org.freedesktop.ModemManager1.Modem.Location.

BUG=chromium-os:37173
TEST=Build and ran unit tests.

Change-Id: I22843677c0e23eb21437a1273cf3ec3f7c6b086b
Reviewed-on: https://gerrit.chromium.org/gerrit/39630
Commit-Ready: Arman Uguray <armansito@chromium.org>
Reviewed-by: Arman Uguray <armansito@chromium.org>
Tested-by: Arman Uguray <armansito@chromium.org>
diff --git a/callbacks.h b/callbacks.h
index 4081054..eb6eb12 100644
--- a/callbacks.h
+++ b/callbacks.h
@@ -19,6 +19,8 @@
                             const Error &)> DBusPropertyMapCallback;
 typedef base::Callback<void(const std::vector<DBusPropertiesMap> &,
                             const Error &)> DBusPropertyMapsCallback;
+typedef base::Callback<void(const DBusEnumValueMap &,
+                            const Error &)> DBusEnumValueMapCallback;
 typedef base::Callback<void(const DBus::Path &,
                             const Error &)> DBusPathCallback;
 typedef base::Callback<void(