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

chrome/browser/pdf/pdf_extension_test.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 <vector>
 
 #include "base/base_paths.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_util.h"
 #include "base/hash.h"
@@ skipping 71 matching lines @@
     };
     for (size_t i = 0; i < arraysize(kFailingPdfs); ++i) {
       if (kFailingPdfs[i] == pdf_file)
         return true;
     }
     return false;
   }
 
   // Runs the extensions test at chrome/test/data/pdf/<filename> on the PDF file
   // at chrome/test/data/pdf/<pdf_filename>.
-  void RunTestsInFile(std::string filename, std::string pdf_filename) {
+  void RunTestsInFile(const std::string& filename,
+                      const std::string& pdf_filename) {
     extensions::ResultCatcher catcher;
 
     GURL url(embedded_test_server()->GetURL("/pdf/" + pdf_filename));
 
     // It should be good enough to just navigate to the URL. But loading up the
     // BrowserPluginGuest seems to happen asynchronously as there was flakiness
     // being seen due to the BrowserPluginGuest not being available yet (see
     // crbug.com/498077). So instead use |LoadPdf| which ensures that the PDF is
     // loaded before continuing.
     ASSERT_TRUE(LoadPdf(url));
@@ skipping 318 matching lines @@
   RunTestsInFile("toolbar_manager_test.js", "test.pdf");
 }
 
 IN_PROC_BROWSER_TEST_F(MaterialPDFExtensionTest, Title) {
   RunTestsInFile("title_test.js", "test-title.pdf");
 }
 
 IN_PROC_BROWSER_TEST_F(MaterialPDFExtensionTest, WhitespaceTitle) {
   RunTestsInFile("whitespace_title_test.js", "test-whitespace-title.pdf");
 }