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_service.cc b/wimax_service.cc
index b34b5bc..588d5ab 100644
--- a/wimax_service.cc
+++ b/wimax_service.cc
@@ -67,6 +67,12 @@
     return false;
   }
 
+  int signal_strength = proxy_->SignalStrength(&error);
+  if (!error.IsSuccess()) {
+    return false;
+  }
+  SetStrength(signal_strength);
+
   set_friendly_name(network_name_);
   storage_id_ =
       StringToLowerASCII(base::StringPrintf("%s_%s_%08x_%s",