Make ExportedObject and ObjectProxy own Bus as scoped_refptr.

dbus/bus.cc

Index: dbus/bus.cc
diff --git a/dbus/bus.cc b/dbus/bus.cc
index 843739ce9e218fc7f313c6536b12995a73eed6f0..598056bf441f36afb04631e7bfd18a222fbf0005 100644
--- a/dbus/bus.cc
+++ b/dbus/bus.cc
@@ -293,6 +293,12 @@ void Bus::ShutdownAndBlock() {
     iter->second->Detach();
   }
 
+  // Release object proxies and exported objects here. We should do this
+  // here rather than in the destructor to avoid memory leaks due to
+  // cyclic references.
+  object_proxy_table_.clear();
+  exported_object_table_.clear();
+
   // Private connection should be closed.
   if (connection_) {
     if (connection_type_ == PRIVATE)