[shill] Enable objects to query an opaque RPC-system ID from Adaptors

This is needed for some properties.  For example Service has a property called Device, which is expected
to return a (in the current impl) a DBus path for the associated Device object.

BUG=chromium-os:16343
TEST=unit tests

Change-Id: I8bd32ab483331efabbfee05dbdeba045c7cb20da
Reviewed-on: http://gerrit.chromium.org/gerrit/3417
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Chris Masone <cmasone@chromium.org>
diff --git a/wifi_service.cc b/wifi_service.cc
index 260245f..6f4e959 100644
--- a/wifi_service.cc
+++ b/wifi_service.cc
@@ -82,4 +82,8 @@
   wifi_->ConnectTo(*this);
 }
 
+string WiFiService::GetDeviceRpcId() {
+  return wifi_->GetRpcIdentifier();
+}
+
 }  // namespace shill