Make RealDevicePolicyProvider own session_manager.

It's the only class using this proxy.

Bug: 28800946
Test: mma

Change-Id: I4a7e5469aa0a27d5756a88a0d6af4017737b70fc
diff --git a/real_system_state.cc b/real_system_state.cc
index 8ab18ff..16d8876 100644
--- a/real_system_state.cc
+++ b/real_system_state.cc
@@ -38,8 +38,7 @@
 namespace chromeos_update_engine {
 
 RealSystemState::RealSystemState(const scoped_refptr<dbus::Bus>& bus)
-    : debugd_proxy_(bus),
-      session_manager_proxy_(bus) {}
+    : debugd_proxy_(bus) {}
 
 RealSystemState::~RealSystemState() {
   // Prevent any DBus communication from UpdateAttempter when shutting down the
@@ -146,7 +145,7 @@
   // Initialize the Update Manager using the default state factory.
   chromeos_update_manager::State* um_state =
       chromeos_update_manager::DefaultStateFactory(
-          &policy_provider_, &session_manager_proxy_, &libcros_proxy_, this);
+          &policy_provider_, &libcros_proxy_, this);
   if (!um_state) {
     LOG(ERROR) << "Failed to initialize the Update Manager.";
     return false;