Make DBus a singleton.

All the proxies need dbus to initialize, so we are passing dbus around,
to make it easier to compile dbus conditionally, this patch makes it a
singleton so that the proxies can get dbus on their own.

Test: mma
Bug: 28800946

Change-Id: Idf062c843aa34a431c2201bae5b895dc1d0ea787
diff --git a/update_engine.gyp b/update_engine.gyp
index 880a03a..d437003 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -254,6 +254,7 @@
         'connection_manager.cc',
         'connection_utils.cc',
         'daemon.cc',
+        'dbus_connection.cc',
         'dbus_service.cc',
         'hardware_chromeos.cc',
         'image_properties_chromeos.cc',