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/dbus_service.h b/dbus_service.h
index 12f8cf1..8b25d43 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -147,8 +147,7 @@
 class UpdateEngineAdaptor : public org::chromium::UpdateEngineInterfaceAdaptor,
                             public ServiceObserverInterface {
  public:
-  UpdateEngineAdaptor(SystemState* system_state,
-                      const scoped_refptr<dbus::Bus>& bus);
+  UpdateEngineAdaptor(SystemState* system_state);
   ~UpdateEngineAdaptor() = default;
 
   // Register the DBus object with the update engine service asynchronously.