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

chrome/browser/chromeos/display/display_preferences.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/display/display_preferences.h"
 
 #include "ash/display/display_layout_store.h"
 #include "ash/display/display_manager.h"
 #include "ash/display/display_pref_util.h"
 #include "ash/display/display_util.h"
@@ skipping 62 matching lines @@
       return "movie";
     case ui::COLOR_PROFILE_READING:
       return "reading";
     case ui::NUM_COLOR_PROFILES:
       break;
   }
   NOTREACHED();
   return "";
 }
 
-ui::ColorCalibrationProfile StringToColorProfile(std::string value) {
+ui::ColorCalibrationProfile StringToColorProfile(const std::string& value) {
   if (value == "standard")
     return ui::COLOR_PROFILE_STANDARD;
   else if (value == "dynamic")
     return ui::COLOR_PROFILE_DYNAMIC;
   else if (value == "movie")
     return ui::COLOR_PROFILE_MOVIE;
   else if (value == "reading")
     return ui::COLOR_PROFILE_READING;
   NOTREACHED();
   return ui::COLOR_PROFILE_STANDARD;
@@ skipping 303 matching lines @@
                                    const ash::DisplayLayout& layout) {
   StoreDisplayLayoutPref(ash::CreateDisplayIdPair(id1, id2), layout);
 }
 
 // Stores the given |power_state|.
 void StoreDisplayPowerStateForTest(DisplayPowerState power_state) {
   StoreDisplayPowerState(power_state);
 }
 
 }  // namespace chromeos