Return const by ref instead of const by value in Issue:id()

chrome/browser/media/router/issue.h

 // Copyright 2015 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 CHROME_BROWSER_MEDIA_ROUTER_ISSUE_H_
 #define CHROME_BROWSER_MEDIA_ROUTER_ISSUE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 55 matching lines @@
 
   // See constructor comments for more information about these fields.
   const std::string& title() const { return title_; }
   const std::string& message() const { return message_; }
   IssueAction default_action() const { return default_action_; }
   const std::vector<IssueAction>& secondary_actions() const {
     return secondary_actions_;
   }
   MediaRoute::Id route_id() const { return route_id_; }
   Severity severity() const { return severity_; }
-  const Issue::Id id() const { return id_; }
+  const Issue::Id& id() const { return id_; }
   bool is_blocking() const { return is_blocking_; }
   bool is_global() const { return route_id_.empty(); }
   const GURL& help_url() const { return help_url_; }
 
   bool Equals(const Issue& other) const;
 
  private:
   std::string title_;
   std::string message_;
   IssueAction default_action_;
   std::vector<IssueAction> secondary_actions_;
   std::string route_id_;
   Severity severity_;
   Issue::Id id_;
   bool is_blocking_;
   GURL help_url_;
 };
 
 }  // namespace media_router
 
 #endif  // CHROME_BROWSER_MEDIA_ROUTER_ISSUE_H_