Return const by ref instead of const by value

Source/core/loader/LinkHeader.h

Index: Source/core/loader/LinkHeader.h
diff --git a/Source/core/loader/LinkHeader.h b/Source/core/loader/LinkHeader.h
index b93ffd816b90c4fdaba630cc4b04167c03640fe6..af14abf0e417ff36d1b8fbb8ad73d4c29061eba1 100644
--- a/Source/core/loader/LinkHeader.h
+++ b/Source/core/loader/LinkHeader.h
@@ -16,8 +16,8 @@ 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; }