Cleanup: Pass std::string as const reference if possible

components/certificate_transparency/log_proof_fetcher_unittest.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/certificate_transparency/log_proof_fetcher.h"
 
 #include <string>
 
 #include "base/strings/stringprintf.h"
 #include "components/safe_json/testing_json_parser.h"
@@ skipping 80 matching lines @@
       net::NetworkDelegate* network_delegate) const override {
     std::string expected_url = base::StringPrintf(
         "%s://%s/%s/ct/v1/get-sth", kLogSchema, kLogHost, kLogPathPrefix);
     EXPECT_EQ(GURL(expected_url), request->url());
     FetchSTHTestJob* job = new FetchSTHTestJob(
         response_body_, response_headers_, request, network_delegate);
     job->set_async_io(async_io_);
     return job;
   }
 
-  void set_response_body(std::string response_body) {
+  void set_response_body(const std::string& response_body) {
     response_body_ = response_body;
   }
 
-  void set_response_headers(std::string response_headers) {
+  void set_response_headers(const std::string& response_headers) {
     response_headers_ = response_headers;
   }
 
   void set_async_io(bool async_io) { async_io_ = async_io; }
 
  private:
   bool async_io_;
   std::string response_body_;
   std::string response_headers_;
 
@@ skipping 182 matching lines @@
   RunFetcherWithCallback(&callback);
 
   ASSERT_TRUE(callback.invoked());
   EXPECT_EQ(net::OK, callback.net_error());
   EXPECT_EQ(net::HTTP_NOT_FOUND, callback.http_response_code());
 }
 
 }  // namespace
 
 }  // namespace certificate_transparency