Pass u2f message callback vector as a const reference

device/u2f/u2f_request.h

Index: device/u2f/u2f_request.h
diff --git a/device/u2f/u2f_request.h b/device/u2f/u2f_request.h
index 902a2ca84d6f51275e2a93b97c9c37a6722dff72..f8d8f3ab8408131c3cdd28f9067ad2587e44b03a 100644
--- a/device/u2f/u2f_request.h
+++ b/device/u2f/u2f_request.h
@@ -14,8 +14,9 @@
 namespace device {
 class U2fRequest : HidService::Observer {
  public:
-  using ResponseCallback = base::Callback<void(U2fReturnCode status_code,
-                                               std::vector<uint8_t> response)>;
+  using ResponseCallback =
+      base::Callback<void(U2fReturnCode status_code,
+                          const std::vector<uint8_t>& response)>;
 
   U2fRequest(const ResponseCallback& callback);
   virtual ~U2fRequest();