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

extensions/common/manifest_test.h

 // 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.
 
 #ifndef EXTENSIONS_COMMON_MANIFEST_TEST_H_
 #define EXTENSIONS_COMMON_MANIFEST_TEST_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/values.h"
@@ skipping 112 matching lines @@
 
   // used to differentiate between calls to LoadAndExpectError,
   // LoadAndExpectWarning and LoadAndExpectSuccess via function RunTestcases.
   enum ExpectType {
     EXPECT_TYPE_ERROR,
     EXPECT_TYPE_WARNING,
     EXPECT_TYPE_SUCCESS
   };
 
   struct Testcase {
-    std::string manifest_filename_;
+    const std::string manifest_filename_;
     std::string expected_error_; // only used for ExpectedError tests
     extensions::Manifest::Location location_;
     int flags_;
 
-    Testcase(std::string manifest_filename, std::string expected_error,
-        extensions::Manifest::Location location, int flags);
+    Testcase(const std::string& manifest_filename,
+             const std::string& expected_error,
+             extensions::Manifest::Location location,
+             int flags);
 
-    Testcase(std::string manifest_filename, std::string expected_error);
+    Testcase(const std::string& manifest_filename,
+             const std::string& expected_error);
 
-    explicit Testcase(std::string manifest_filename);
+    explicit Testcase(const std::string& manifest_filename);
 
-    Testcase(std::string manifest_filename,
+    Testcase(const std::string& manifest_filename,
              extensions::Manifest::Location location,
              int flags);
   };
 
   void RunTestcases(const Testcase* testcases,
                     size_t num_testcases,
                     ExpectType type);
 
   void RunTestcase(const Testcase& testcase, ExpectType type);
 
   bool enable_apps_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ManifestTest);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_MANIFEST_TEST_H_