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/real_system_state.cc b/real_system_state.cc
index 8a5f389..a71f4cd 100644
--- a/real_system_state.cc
+++ b/real_system_state.cc
@@ -40,10 +40,7 @@
 RealSystemState::RealSystemState(const scoped_refptr<dbus::Bus>& bus)
     : debugd_proxy_(bus),
       power_manager_proxy_(bus),
-      session_manager_proxy_(bus),
-      shill_proxy_(bus),
-      libcros_proxy_(bus) {
-}
+      session_manager_proxy_(bus) {}
 
 RealSystemState::~RealSystemState() {
   // Prevent any DBus communication from UpdateAttempter when shutting down the