Pass some arguments by const reference.

Test: Rely on TreeHugger.
Bug: 32619234
Change-Id: Ia26277893641dcb3562eaeacaec4e089a978d56a
diff --git a/runtime/verifier/method_verifier.cc b/runtime/verifier/method_verifier.cc
index 97bc79c..d9e3ea7 100644
--- a/runtime/verifier/method_verifier.cc
+++ b/runtime/verifier/method_verifier.cc
@@ -956,7 +956,7 @@
   delete last_fail_message;
 }
 
-void MethodVerifier::AppendToLastFailMessage(std::string append) {
+void MethodVerifier::AppendToLastFailMessage(const std::string& append) {
   size_t failure_num = failure_messages_.size();
   DCHECK_NE(failure_num, 0U);
   std::ostringstream* last_fail_message = failure_messages_[failure_num - 1];