Return const by ref instead of const by value

Source/core/loader/LinkHeader.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 LinkHeader_h
 #define LinkHeader_h
 
 #include "core/CoreExport.h"
 #include "core/html/CrossOriginAttribute.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 class LinkHeader {
 public:
     template <typename CharType>
     LinkHeader(CharType*& position, CharType* end);
 
-    const String url() const { return m_url; }
-    const String rel() const { return m_rel; }
+    const String& url() const { return m_url; }
+    const String& rel() const { return m_rel; }
     CrossOriginAttributeValue crossOrigin() const { return m_crossOrigin; }
     bool valid() const { return m_isValid; }
 
     enum LinkParameterName {
         LinkParameterUnknown,
         LinkParameterRel,
         LinkParameterAnchor,
         LinkParameterCrossOrigin,
     };
 
@@ skipping 18 matching lines @@
 private:
     template <typename CharType>
     void init(CharType* headerValue, unsigned len);
 
     Vector<LinkHeader> m_headerSet;
 };
 
 }
 
 #endif