shill: wimax: Add SignalStrength property to WiMaxNetworkProxy.

BUG=chrome-os-partner:9831
TEST=Build and run unit tests.

Change-Id: If17118658dca8cfdfe60e1047adf51db146c5d7f
Reviewed-on: https://gerrit.chromium.org/gerrit/23099
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
diff --git a/wimax_network_proxy_interface.h b/wimax_network_proxy_interface.h
index 4c82e61..3777ba6 100644
--- a/wimax_network_proxy_interface.h
+++ b/wimax_network_proxy_interface.h
@@ -28,6 +28,7 @@
   virtual int Type(Error *error) = 0;
   virtual int CINR(Error *error) = 0;
   virtual int RSSI(Error *error) = 0;
+  virtual int SignalStrength(Error *error) = 0;
 };
 
 }  // namespace shill