shill: tighten up constness of PropertyStore

BUG=chromium-os:20543
TEST=unittest

Change-Id: Ifa86f82a883b3c3ec61ce10f8d82564b9a036b04
Reviewed-on: http://gerrit.chromium.org/gerrit/7901
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
diff --git a/device_dbus_adaptor.cc b/device_dbus_adaptor.cc
index 31ced31..1b640c2 100644
--- a/device_dbus_adaptor.cc
+++ b/device_dbus_adaptor.cc
@@ -67,7 +67,7 @@
 void DeviceDBusAdaptor::SetProperty(const string& name,
                                     const ::DBus::Variant& value,
                                     ::DBus::Error &error) {
-  DBusAdaptor::DispatchOnType(device_->store(), name, value, &error);
+  DBusAdaptor::DispatchOnType(device_->mutable_store(), name, value, &error);
 }
 
 void DeviceDBusAdaptor::ClearProperty(const std::string& ,