dbus: Add ObjectProxy::EmptyResponseCallback().

dbus/object_proxy.h

Index: dbus/object_proxy.h
diff --git a/dbus/object_proxy.h b/dbus/object_proxy.h
index 5fc41979a8faa3d70794fbf6352a9b6323fb425c..484f107890c94b4f7c6f867d9a39a21040346cf8 100644
--- a/dbus/object_proxy.h
+++ b/dbus/object_proxy.h
@@ -73,7 +73,9 @@ class ObjectProxy : public base::RefCountedThreadSafe<ObjectProxy> {
   // |callback| will be called in the origin thread, once the method call
   // is complete. As it's called in the origin thread, |callback| can
   // safely reference objects in the origin thread (i.e. UI thread in most
-  // cases).
+  // cases). If the caller is not interested in the response from the
+  // method (i.e. calling a method that does not return a value),
+  // EmptyResponseCallback() can be passed to the |callback| parameter.
   //
   // If the method call is successful, a pointer to Response object will
   // be passed to the callback. If unsuccessful, NULL will be passed to
@@ -106,6 +108,10 @@ class ObjectProxy : public base::RefCountedThreadSafe<ObjectProxy> {
   // BLOCKING CALL.
   virtual void Detach();
 
+  // Returns an empty callback that does nothing. Can be used for
+  // CallMethod().
+  static ResponseCallback EmptyResponseCallback();
+
  protected:
   // This is protected, so we can define sub classes.
   virtual ~ObjectProxy();