Add BusConnection::g_connection() function, just like the Proxy class. This change is for http://codereview.chromium.org/521058.

Review URL: http://codereview.chromium.org/527029
diff --git a/chromeos/dbus/dbus.h b/chromeos/dbus/dbus.h
index f11a7ef..81840c5 100644
--- a/chromeos/dbus/dbus.h
+++ b/chromeos/dbus/dbus.h
@@ -46,6 +46,11 @@
     return *this;
   }
 
+  const value_type& g_connection() const {
+    DCHECK(object_) << "referencing an empty connection";
+    return object_;
+  }
+
  private:
   friend void swap(BusConnection& x, BusConnection& y);