dbus: don't fail when reconnecting object signals

dbus/object_proxy.h

Index: dbus/object_proxy.h
diff --git a/dbus/object_proxy.h b/dbus/object_proxy.h
index 3a9fab1b83f3d10a1a9b38d655f85a81fb92b358..6c786ab487c35fe43233acbb13c190a5f84b6be9 100644
--- a/dbus/object_proxy.h
+++ b/dbus/object_proxy.h
@@ -29,7 +29,9 @@ class Signal;
 // calling methods of these objects.
 //
 // ObjectProxy is a ref counted object, to ensure that |this| of the
-// object is is alive when callbacks referencing |this| are called.
+// object is is alive when callbacks referencing |this| are called; the
+// bus always holds at least one of those references so object proxies
+// always last as long as the bus that created them.
 class ObjectProxy : public base::RefCountedThreadSafe<ObjectProxy> {
  public:
   // Client code should use Bus::GetObjectProxy() or
@@ -96,7 +98,8 @@ class ObjectProxy : public base::RefCountedThreadSafe<ObjectProxy> {
                           int timeout_ms,
                           ResponseCallback callback);
 
-  // Requests to connect to the signal from the remote object.
+  // Requests to connect to the signal from the remote object, replacing
+  // any previous |signal_callback| connected to that signal.
   //
   // |signal_callback| will be called in the origin thread, when the
   // signal is received from the remote object. As it's called in the