update_engine: Convert NetworkConnectionType to enum class.

This patch simply converts the old NetworkConnectionType enum to an
enum class and updates all the callsites and formatting.

BUG=None
TEST=emerge-link update_engine.

Change-Id: I29a281f1648e2d8ef74b1046d01a212022280461
Reviewed-on: https://chromium-review.googlesource.com/289585
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Trybot-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/dbus_service.cc b/dbus_service.cc
index a7ec259..0dbff6b 100644
--- a/dbus_service.cc
+++ b/dbus_service.cc
@@ -425,7 +425,7 @@
   // updates. A log message could be printed as the result of this test.
   LOG(INFO) << "Checking if updates over cellular networks are allowed:";
   *allowed = cm->IsUpdateAllowedOver(
-      chromeos_update_engine::kNetCellular,
+      chromeos_update_engine::NetworkConnectionType::kCellular,
       chromeos_update_engine::NetworkTethering::kUnknown);
 
   return TRUE;