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

chrome/browser/media_galleries/media_galleries_scan_result_controller.h

 // 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.
 
 #ifndef CHROME_BROWSER_MEDIA_GALLERIES_MEDIA_GALLERIES_SCAN_RESULT_CONTROLLER_H_
 #define CHROME_BROWSER_MEDIA_GALLERIES_MEDIA_GALLERIES_SCAN_RESULT_CONTROLLER_H_
 
 #include <map>
 #include <set>
 #include <string>
@@ skipping 84 matching lines @@
   friend class MediaGalleriesScanResultCocoaTest;
   friend class TestMediaGalleriesAddScanResultsFunction;
 
   // Bottom half of constructor -- called when |preferences_| is initialized.
   void OnPreferencesInitialized();
 
   // Used to keep the dialog in sync with the preferences.
   void OnPreferenceUpdate(const std::string& extension_id);
 
   // Used to keep the dialog in sync with attached and detached devices.
-  void OnRemovableDeviceUpdate(const std::string device_id);
+  void OnRemovableDeviceUpdate(const std::string& device_id);
 
   Profile* GetProfile() const;
 
   // RemovableStorageObserver implementation.
   // Used to keep dialog in sync with removable device status.
   void OnRemovableStorageAttached(
       const storage_monitor::StorageInfo& info) override;
   void OnRemovableStorageDetached(
       const storage_monitor::StorageInfo& info) override;
 
@@ skipping 39 matching lines @@
 
   // The view that's showing.
   scoped_ptr<MediaGalleriesDialog> dialog_;
 
   scoped_ptr<MediaGalleryContextMenu> context_menu_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaGalleriesScanResultController);
 };
 
 #endif  // CHROME_BROWSER_MEDIA_GALLERIES_MEDIA_GALLERIES_SCAN_RESULT_CONTROLLER_H_