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

ios/crnet/crnet_environment.mm

 // 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.
 
 #include "ios/crnet/crnet_environment.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/at_exit.h"
 #include "base/command_line.h"
@@ skipping 235 matching lines @@
       GetHttpNetworkSession(GetMainContextGetter()->GetURLRequestContext());
 
   if (http_network_session) {
     net::SpdySessionPool *spdy_session_pool =
         http_network_session->spdy_session_pool();
     if (spdy_session_pool)
       spdy_session_pool->CloseCurrentSessions(net::ERR_ABORTED);
   }
 }
 
-CrNetEnvironment::CrNetEnvironment(std::string user_agent_product_name)
+CrNetEnvironment::CrNetEnvironment(const std::string& user_agent_product_name)
     : spdy_enabled_(false),
       quic_enabled_(false),
       sdch_enabled_(false),
       main_context_(new net::URLRequestContext),
       user_agent_product_name_(user_agent_product_name),
-      net_log_(new net::NetLog) {
-
-}
+      net_log_(new net::NetLog) {}
 
 void CrNetEnvironment::Install() {
   // Threads setup.
   network_cache_thread_.reset(new base::Thread("Chrome Network Cache Thread"));
   network_cache_thread_->StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
   network_io_thread_.reset(new base::Thread("Chrome Network IO Thread"));
   network_io_thread_->StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
   file_thread_.reset(new base::Thread("Chrome File Thread"));
@@ skipping 228 matching lines @@
       if (backend)
         backend->DoomAllEntries(client_callback);
   });
   int rc = cache->GetBackend(&backend, doom_callback);
   if (rc != net::ERR_IO_PENDING) {
     // GetBackend doesn't call the callback if it completes synchronously, so
     // call it directly here.
     doom_callback.Run(rc);
   }
 }