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

chrome/browser/media/webrtc_browsertest_base.h

 // Copyright 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.
 
 #ifndef CHROME_BROWSER_MEDIA_WEBRTC_BROWSERTEST_BASE_H_
 #define CHROME_BROWSER_MEDIA_WEBRTC_BROWSERTEST_BASE_H_
 
 #include <string>
 
 #include "chrome/test/base/in_process_browser_test.h"
@@ skipping 123 matching lines @@
   // Returns true if we're on win 8.
   bool OnWin8() const;
 
  private:
   void CloseInfoBarInTab(content::WebContents* tab_contents,
                          infobars::InfoBar* infobar) const;
 
   std::string CreateLocalOffer(content::WebContents* from_tab) const;
   std::string CreateAnswer(std::string local_offer,
                            content::WebContents* to_tab) const;
-  void ReceiveAnswer(std::string answer, content::WebContents* from_tab) const;
+  void ReceiveAnswer(const std::string& answer,
+                     content::WebContents* from_tab) const;
   void GatherAndSendIceCandidates(content::WebContents* from_tab,
                                   content::WebContents* to_tab) const;
 
   infobars::InfoBar* GetUserMediaAndWaitForInfoBar(
       content::WebContents* tab_contents,
       const std::string& constraints) const;
 
   bool detect_errors_in_javascript_;
 
   DISALLOW_COPY_AND_ASSIGN(WebRtcTestBase);
 };
 
 #endif  // CHROME_BROWSER_MEDIA_WEBRTC_BROWSERTEST_BASE_H_