[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/manager_dbus_adaptor.h b/manager_dbus_adaptor.h
index 6a9e0e4..62b8579 100644
--- a/manager_dbus_adaptor.h
+++ b/manager_dbus_adaptor.h
@@ -35,6 +35,7 @@
   virtual ~ManagerDBusAdaptor();
 
   // Implementation of ManagerAdaptorInterface.
+  virtual const std::string &GetRpcIdentifier() { return path(); }
   void UpdateRunning();
   void EmitBoolChanged(const std::string &name, bool value);
   void EmitUintChanged(const std::string &name, uint32 value);