[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/ethernet_service.h b/ethernet_service.h
index df708ae..53cc021 100644
--- a/ethernet_service.h
+++ b/ethernet_service.h
@@ -7,7 +7,6 @@
 
 #include <base/basictypes.h>
 
-#include "shill/ethernet.h"
 #include "shill/refptr_types.h"
 #include "shill/shill_event.h"
 #include "shill/service.h"
@@ -28,6 +27,8 @@
   virtual std::string CalculateState() { return "idle"; }
 
  private:
+  std::string GetDeviceRpcId();
+
   EthernetRefPtr ethernet_;
   const std::string type_;
   DISALLOW_COPY_AND_ASSIGN(EthernetService);