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

chrome/browser/chromeos/preferences_unittest.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/browser/chromeos/preferences.h"
 
 #include "base/json/json_string_value_serializer.h"
 #include "base/prefs/pref_member.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
@@ skipping 321 matching lines @@
   void ExpectGlobalValues(const std::string& preferred_languages,
                           const std::string& preload_engines,
                           const std::string& enabled_extension_imes) {
     EXPECT_EQ(preferred_languages, preferred_languages_syncable_.GetValue());
     EXPECT_EQ(preload_engines, preload_engines_syncable_.GetValue());
     EXPECT_EQ(enabled_extension_imes,
               enabled_extension_imes_syncable_.GetValue());
   }
 
   // Translates engine IDs in a CSV string to input method IDs.
-  std::string ToInputMethodIds(std::string value) {
+  std::string ToInputMethodIds(const std::string& value) {
     std::vector<std::string> tokens = base::SplitString(
         value, ",", base::TRIM_WHITESPACE, base::SPLIT_WANT_ALL);
     std::transform(tokens.begin(), tokens.end(), tokens.begin(),
                    &extension_ime_util::GetInputMethodIDByEngineID);
     return base::JoinString(tokens, ",");
   }
 
   StringPrefMember preferred_languages_;
   StringPrefMember preferred_languages_syncable_;
   StringPrefMember preload_engines_;
@@ skipping 345 matching lines @@
         "Syncable values should have added local values on initialization.");
     ExpectGlobalValues(
         "es,en-US",
         "xkb:es::spa,xkb:us::eng,xkb:ru::rus,xkb:xy::xyz," +
             ToInputMethodIds("xkb:jp::jpn"),
         std::string(kIdentityIMEID) + "," + kUnknownIMEID);
   }
 }
 
 }  // namespace chromeos