Coverity: Fix pass by value error in SpeechRecognitionHypothesis constructor.

content/public/common/speech_recognition_result.h

 // Copyright (c) 2012 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 CONTENT_PUBLIC_COMMON_SPEECH_RECOGNITION_RESULT_H_
 #define CONTENT_PUBLIC_COMMON_SPEECH_RECOGNITION_RESULT_H_
 #pragma once
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/string16.h"
 #include "content/common/content_export.h"
 
 namespace content {
 
 struct SpeechRecognitionHypothesis {
   string16 utterance;
   double confidence;
 
   SpeechRecognitionHypothesis() : confidence(0.0) {}
 
-  SpeechRecognitionHypothesis(const string16 utterance_value,
+  SpeechRecognitionHypothesis(const string16& utterance_value,
                               double confidence_value)
       : utterance(utterance_value),
         confidence(confidence_value) {
   }
 };
 
 typedef std::vector<SpeechRecognitionHypothesis>
     SpeechRecognitionHypothesisArray;
 
 struct CONTENT_EXPORT SpeechRecognitionResult {
   SpeechRecognitionHypothesisArray hypotheses;
 
   SpeechRecognitionResult();
   ~SpeechRecognitionResult();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_COMMON_SPEECH_RECOGNITION_RESULT_H_