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

chrome/browser/pdf/pdf_extension_test.cc

Index: chrome/browser/pdf/pdf_extension_test.cc
diff --git a/chrome/browser/pdf/pdf_extension_test.cc b/chrome/browser/pdf/pdf_extension_test.cc
index 3ddc059fda093f979b65af8ef5980e6e922f2263..6181f41ac886cb876915822cd3d4722fd6fee921 100644
--- a/chrome/browser/pdf/pdf_extension_test.cc
+++ b/chrome/browser/pdf/pdf_extension_test.cc
@@ -89,7 +89,8 @@ class PDFExtensionTest : public ExtensionApiTest,
 
   // 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));