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

chrome/browser/tracing/crash_service_uploader.cc

 // 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.
 
 #include "chrome/browser/tracing/crash_service_uploader.h"
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/format_macros.h"
@@ skipping 176 matching lines @@
   std::string post_data;
   SetupMultipart(product, version, metadata.Pass(), "trace.json.gz",
                  compressed_contents, &post_data);
 
   content::BrowserThread::PostTask(
       content::BrowserThread::UI, FROM_HERE,
       base::Bind(&TraceCrashServiceUploader::CreateAndStartURLFetcher,
                  base::Unretained(this), upload_url, post_data));
 }
 
-void TraceCrashServiceUploader::OnUploadError(std::string error_message) {
+void TraceCrashServiceUploader::OnUploadError(
+    const std::string& error_message) {
   LOG(ERROR) << error_message;
   content::BrowserThread::PostTask(
       content::BrowserThread::UI, FROM_HERE,
       base::Bind(done_callback_, false, error_message));
 }
 
 void TraceCrashServiceUploader::SetupMultipart(
     const std::string& product,
     const std::string& version,
     scoped_ptr<base::DictionaryValue> metadata,
@@ skipping 86 matching lines @@
   std::string content_type = kUploadContentType;
   content_type.append("; boundary=");
   content_type.append(kMultipartBoundary);
 
   url_fetcher_ =
       net::URLFetcher::Create(GURL(upload_url), net::URLFetcher::POST, this);
   url_fetcher_->SetRequestContext(request_context_);
   url_fetcher_->SetUploadData(content_type, post_data);
   url_fetcher_->Start();
 }