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

chromeos/dbus/cashew_client.h

Index: chromeos/dbus/cashew_client.h
diff --git a/chromeos/dbus/cashew_client.h b/chromeos/dbus/cashew_client.h
index 2238e1bc855adb2b11b91214d26fc869585c9474..195f25631f94a874ffda8dd7b422ba209c3e631f 100644
--- a/chromeos/dbus/cashew_client.h
+++ b/chromeos/dbus/cashew_client.h
@@ -44,7 +44,8 @@ class CHROMEOS_EXPORT CashewClient {
                               dbus::Bus* bus);
 
   // Sets DataPlansUpdate signal handler.
-  virtual void SetDataPlansUpdateHandler(DataPlansUpdateHandler handler) = 0;
+  virtual void SetDataPlansUpdateHandler(
+      const DataPlansUpdateHandler& handler) = 0;
 
   // Resets DataPlansUpdate signal handler.
   virtual void ResetDataPlansUpdateHandler() = 0;