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

chrome/browser/ui/cocoa/profiles/user_manager_mac.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 "chrome/browser/ui/cocoa/profiles/user_manager_mac.h"
 
 #include "base/mac/foundation_util.h"
 #include "chrome/app/chrome_command_ids.h"
 #import "chrome/browser/app_controller_mac.h"
 #include "chrome/browser/browser_process.h"
@@ skipping 124 matching lines @@
         isTextEditingCommand) {
       [[NSApp mainMenu] performKeyEquivalent:event.os_event];
     }
   }
 };
 
 class ReauthDialogDelegate : public UserManager::ReauthDialogObserver,
                              public UserManagerWebContentsDelegate,
                              public ConstrainedWindowMacDelegate {
  public:
-   ReauthDialogDelegate(content::WebContents* web_contents, std::string email)
+  ReauthDialogDelegate(content::WebContents* web_contents,
+                       const std::string& email)
       : UserManager::ReauthDialogObserver(web_contents, email) {}
 
   // UserManager::ReauthDialogObserver:
   void CloseReauthDialog() override {
     CloseInstanceReauthDialog();
   }
 
   // ConstrainedWindowMacDelegate:
   void OnConstrainedWindowClosed(ConstrainedWindowMac* window) override {
     CloseReauthDialog();
@@ skipping 315 matching lines @@
   ProfileMetrics::LogTimeToOpenUserManager(
       base::Time::Now() - user_manager_started_showing_);
   user_manager_started_showing_ = base::Time();
 }
 
 void UserManagerMac::WindowWasClosed() {
   CloseReauthDialog();
   instance_ = NULL;
   delete this;
 }