Coverity: Fix a few pass-by-values.

chrome/common/extensions/extension_file_util.cc

 // Copyright (c) 2012 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/common/extensions/extension_file_util.h"
 
 #include <map>
 #include <vector>
 
 #include "base/file_util.h"
@@ skipping 111 matching lines @@
 
 scoped_refptr<Extension> LoadExtension(const FilePath& extension_path,
                                        Extension::Location location,
                                        int flags,
                                        std::string* error) {
   return LoadExtension(
       extension_path, std::string(), location, flags, error);
 }
 
 scoped_refptr<Extension> LoadExtension(const FilePath& extension_path,
-                                       std::string extension_id,
+                                       const std::string& extension_id,
                                        Extension::Location location,
                                        int flags,
                                        std::string* error) {
   scoped_ptr<DictionaryValue> manifest(LoadManifest(extension_path, error));
   if (!manifest.get())
     return NULL;
   if (!extension_l10n_util::LocalizeExtension(extension_path, manifest.get(),
                                               error))
     return NULL;
 
@@ skipping 544 matching lines @@
     return temp_path;
 
   return FilePath();
 }
 
 void DeleteFile(const FilePath& path, bool recursive) {
   file_util::Delete(path, recursive);
 }
 
 }  // namespace extension_file_util