chromeos: Convert D-Bus client classes' callback arguments to const reference

chromeos/dbus/image_burner_client.h

Index: chromeos/dbus/image_burner_client.h
diff --git a/chromeos/dbus/image_burner_client.h b/chromeos/dbus/image_burner_client.h
index 513496d34cb005fd74272b4856a271933b533e74..5368eb00dfe874b1b98992554319ad9c3eef4885 100644
--- a/chromeos/dbus/image_burner_client.h
+++ b/chromeos/dbus/image_burner_client.h
@@ -42,15 +42,15 @@ class CHROMEOS_EXPORT ImageBurnerClient {
   // Burns the image |from_path| to the disk |to_path|.
   virtual void BurnImage(const std::string& from_path,
                          const std::string& to_path,
-                         ErrorCallback error_callback) = 0;
+                         const ErrorCallback& error_callback) = 0;
 
   // Sets callbacks as event handlers.
   // |burn_finished_handler| is called when burn_finished signal is received.
   // |burn_progress_update_handler| is called when burn_progress_update signal
   // is received.
   virtual void SetEventHandlers(
-      BurnFinishedHandler burn_finished_handler,
-      BurnProgressUpdateHandler burn_progress_update_handler) = 0;
+      const BurnFinishedHandler& burn_finished_handler,
+      const BurnProgressUpdateHandler& burn_progress_update_handler) = 0;
 
   // Resets event handlers. After calling this method, nothing is done when
   // signals are received.