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/connection_manager.h b/connection_manager.h
index f9973a3..687759e 100644
--- a/connection_manager.h
+++ b/connection_manager.h
@@ -11,13 +11,13 @@
 
 namespace chromeos_update_engine {
 
-enum NetworkConnectionType {
-  kNetEthernet = 0,
-  kNetWifi,
-  kNetWimax,
-  kNetBluetooth,
-  kNetCellular,
-  kNetUnknown
+enum class NetworkConnectionType {
+  kEthernet = 0,
+  kWifi,
+  kWimax,
+  kBluetooth,
+  kCellular,
+  kUnknown
 };
 
 enum class NetworkTethering {