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

chrome/browser/extensions/extension_message_bubble_controller.cc

 // Copyright (c) 2013 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/extensions/extension_message_bubble_controller.h"
 
 #include "base/bind.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
@@ skipping 65 matching lines @@
                              pref_name,
                              value ? new base::FundamentalValue(value) : NULL);
 }
 
 std::string
 ExtensionMessageBubbleController::Delegate::get_acknowledged_flag_pref_name()
     const {
   return acknowledged_pref_name_;
 }
 
-void
-ExtensionMessageBubbleController::Delegate::set_acknowledged_flag_pref_name(
-    std::string pref_name) {
+void ExtensionMessageBubbleController::Delegate::
+    set_acknowledged_flag_pref_name(const std::string& pref_name) {
   acknowledged_pref_name_ = pref_name;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // ExtensionMessageBubbleController
 
 ExtensionMessageBubbleController::ExtensionMessageBubbleController(
     Delegate* delegate,
     Browser* browser)
     : browser_(browser),
@@ skipping 146 matching lines @@
   return &extension_list_;
 }
 
 void ExtensionMessageBubbleController::OnClose() {
   AcknowledgeExtensions();
   if (did_highlight_)
     ToolbarActionsModel::Get(profile())->StopHighlighting();
 }
 
 }  // namespace extensions