shill: Add Manager::GetDefaultService

Also fix SortServices to fix reporting of "DefaultService" to metrics.

BUG=chromium-os:27483
TEST=New unit tests

Change-Id: Ic456e51de366ac4181bcd32d49d6e4055622c1cc
Reviewed-on: https://gerrit.chromium.org/gerrit/18133
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
diff --git a/mock_manager.h b/mock_manager.h
index 9b74211..4e66453 100644
--- a/mock_manager.h
+++ b/mock_manager.h
@@ -31,6 +31,7 @@
   MOCK_METHOD2(HandleProfileEntryDeletion,
                bool (const ProfileRefPtr &profile,
                      const std::string &entry_name));
+  MOCK_CONST_METHOD0(GetDefaultService, ServiceRefPtr());
   MOCK_METHOD1(IsPortalDetectionEnabled, bool(Technology::Identifier tech));
   MOCK_CONST_METHOD0(GetPortalCheckURL, const std::string &());
   MOCK_CONST_METHOD0(GetPortalCheckInterval, int());