shill: Add support for saving wifi frequency in service object.

BUG=chromium-os:25882
TEST=Unit tests, network_WiFiManager (7/9)

Change-Id: Ia491ef6b240778781470d7ff820e23deb7aa28f0
Reviewed-on: https://gerrit.chromium.org/gerrit/15363
Commit-Ready: Thieu Le <thieule@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
diff --git a/wifi.h b/wifi.h
index 442e93d..3793f91 100644
--- a/wifi.h
+++ b/wifi.h
@@ -67,6 +67,7 @@
  private:
   friend class WiFiMainTest;  // access to supplicant_*_proxy_, link_up_
   FRIEND_TEST(WiFiMainTest, InitialSupplicantState);  // kInterfaceStateUnknown
+  FRIEND_TEST(WiFiMainTest, ScanResults);             // EndpointMap
   FRIEND_TEST(WiFiMainTest, ScanResultsWithUpdates);  // EndpointMap
   FRIEND_TEST(WiFiPropertyTest, ClearDerivedProperty);  // bgscan_method_