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.cc b/wimax_network_proxy.cc
index 9859d67..c0581be 100644
--- a/wimax_network_proxy.cc
+++ b/wimax_network_proxy.cc
@@ -74,6 +74,16 @@
   return 0;
 }
 
+int WiMaxNetworkProxy::SignalStrength(Error *error) {
+  SLOG(DBus, 2) << __func__;
+  try {
+    return proxy_.SignalStrength();
+  } catch (const DBus::Error &e) {
+    FromDBusError(e, error);
+  }
+  return 0;
+}
+
 // static
 void WiMaxNetworkProxy::FromDBusError(const DBus::Error &dbus_error,
                                       Error *error) {