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

chrome/browser/tracing/crash_service_uploader.h

 // Copyright 2014 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.
 
 #ifndef CHROME_BROWSER_TRACING_CRASH_SERVICE_UPLOADER_H_
 #define CHROME_BROWSER_TRACING_CRASH_SERVICE_UPLOADER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 59 matching lines @@
   void AddTraceFile(const std::string& trace_filename,
                     const std::string& trace_contents,
                     std::string* post_data);
   // Compresses the input and returns whether compression was successful.
   bool Compress(std::string input,
                 int max_compressed_bytes,
                 char* compressed_contents,
                 int* compressed_bytes);
   void CreateAndStartURLFetcher(const std::string& upload_url,
                                 const std::string& post_data);
-  void OnUploadError(std::string error_message);
+  void OnUploadError(const std::string& error_message);
 
   scoped_ptr<net::URLFetcher> url_fetcher_;
   UploadProgressCallback progress_callback_;
   UploadDoneCallback done_callback_;
 
   net::URLRequestContextGetter* request_context_;
 
   std::string upload_url_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceCrashServiceUploader);
 };
 
 #endif  // CHROME_BROWSER_TRACING_CRASH_SERVICE_UPLOADER_H_