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

extensions/browser/api/declarative/rules_registry_service.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_BROWSER_API_DECLARATIVE_RULES_REGISTRY_SERVICE_H__
 #define EXTENSIONS_BROWSER_API_DECLARATIVE_RULES_REGISTRY_SERVICE_H__
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 22 matching lines @@
 // This class lives on the UI thread.
 class RulesRegistryService : public BrowserContextKeyedAPI,
                              public ExtensionRegistryObserver {
  public:
   static const int kDefaultRulesRegistryID;
   static const int kInvalidRulesRegistryID;
 
   struct RulesRegistryKey {
     std::string event_name;
     int rules_registry_id;
-    RulesRegistryKey(const std::string event_name, int rules_registry_id)
-        : event_name(event_name),
-          rules_registry_id(rules_registry_id) {}
+    RulesRegistryKey(const std::string& event_name, int rules_registry_id)
+        : event_name(event_name), rules_registry_id(rules_registry_id) {}
     bool operator<(const RulesRegistryKey& other) const {
       return (event_name < other.event_name) ||
              ((event_name == other.event_name) &&
               (rules_registry_id < other.rules_registry_id));
     }
   };
 
   explicit RulesRegistryService(content::BrowserContext* context);
   ~RulesRegistryService() override;
 
@@ skipping 89 matching lines @@
       extension_registry_observer_;
 
   content::BrowserContext* browser_context_;
 
   DISALLOW_COPY_AND_ASSIGN(RulesRegistryService);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_API_DECLARATIVE_RULES_REGISTRY_SERVICE_H__