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

ios/crnet/crnet_environment.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 IOS_CRNET_CRNET_ENVIRONMENT_H_
 #define IOS_CRNET_CRNET_ENVIRONMENT_H_
 
 #include "base/files/file_path.h"
 #include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 18 matching lines @@
 class CrNetHttpProtocolHandlerDelegate;
 
 // CrNetEnvironment contains all the network stack configuration
 // and initialization.
 class CrNetEnvironment {
  public:
   // Must be called on the main thread.
   static void Initialize();
 
   // |user_agent_product_name| will be used to generate the user-agent.
-  CrNetEnvironment(std::string user_agent_product_name);
+  CrNetEnvironment(const std::string& user_agent_product_name);
   ~CrNetEnvironment();
 
   // Installs this CrNet environment so requests are intercepted.
   // Can only be called once; to enable/disable CrNet at runtime, use
   // SetHTTPProtocolHandlerRegistered.
   void Install();
 
   // Installs this CrNet environment into the supplied
   // NSURLSessionConfiguration. Settings are inherited from the shared
   // NSURLSessionConfiguration, which Install() affects.
@@ skipping 111 matching lines @@
   scoped_ptr<net::URLRequestContext> main_context_;
   scoped_ptr<CrNetHttpProtocolHandlerDelegate> http_protocol_handler_delegate_;
   std::string user_agent_product_name_;
   scoped_ptr<net::NetLog> net_log_;
   scoped_ptr<net::WriteToFileNetLogObserver> net_log_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(CrNetEnvironment);
 };
 
 #endif  // IOS_CRNET_CRNET_ENVIRONMENT_H_