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

extensions/browser/api/networking_config/networking_config_service.h

 // Copyright 2015 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_BROWSER_API_NETWORKING_CONFIG_NETWORKING_CONFIG_SERVICE_H_
 #define EXTENSIONS_BROWSER_API_NETWORKING_CONFIG_NETWORKING_CONFIG_SERVICE_H_
 
 #include <map>
 #include <string>
 
@@ skipping 52 matching lines @@
                            const Extension* extension,
                            UnloadedExtensionInfo::Reason reason) override;
 
   // Returns the extension id registered for |hex_ssid|. If no extension is
   // registered for this |hex_ssid|, the function returns an empty string.
   // |hex_ssid|: SSID in hex encoding.
   std::string LookupExtensionIdForHexSsid(std::string hex_ssid) const;
 
   // Returns true if the extension with id |extension_id| registered for
   // |onCaptivePortalDetected| events, otherwise false.
-  bool IsRegisteredForCaptivePortalEvent(std::string extension_id) const;
+  bool IsRegisteredForCaptivePortalEvent(const std::string& extension_id) const;
 
   // Registers |hex_ssid| as being handled by the extension with extension ID
   // |extension_id|. Returns true on success and false if another extension
   // already registered for |hex_ssid|.
   // |hex_ssid|:     SSID in hex encoding of the network to be registered.
   // |extension_id|: Extension ID of the extension handling the network
   //                 configuration for this network.
   bool RegisterHexSsid(std::string hex_ssid, const std::string& extension_id);
 
   // Unregisters extension with the ID |extension_id| removing all associated
@@ skipping 55 matching lines @@
 
   // This map associates a given hex encoded SSID to an extension entry.
   std::map<std::string, std::string> hex_ssid_to_extension_id_;
 
   base::WeakPtrFactory<NetworkingConfigService> weak_factory_;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_API_NETWORKING_CONFIG_NETWORKING_CONFIG_SERVICE_H_