[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/device.h b/device.h
index ef75711..641793a 100644
--- a/device.h
+++ b/device.h
@@ -80,10 +80,13 @@
   // instance.
   const std::string &UniqueName() const;
 
+  std::string GetRpcIdentifier();
+
  protected:
   FRIEND_TEST(DeviceTest, AcquireDHCPConfig);
   FRIEND_TEST(DeviceTest, DestroyIPConfig);
   FRIEND_TEST(DeviceTest, DestroyIPConfigNULL);
+  FRIEND_TEST(DeviceTest, GetProperties);
 
   // If there's an IP configuration in |ipconfig_|, releases the IP address and
   // destroys the configuration instance.