Get rid of code duplication in LinkRelAttribute

Source/core/html/LinkRelAttribute.h

Index: Source/core/html/LinkRelAttribute.h
diff --git a/Source/core/html/LinkRelAttribute.h b/Source/core/html/LinkRelAttribute.h
index 9c2a147225e1f8e8bb0d2450f2b726c65b820683..ec04cb3f0b63f43bf5194a1a65fe56f4b7282228 100644
--- a/Source/core/html/LinkRelAttribute.h
+++ b/Source/core/html/LinkRelAttribute.h
@@ -33,17 +33,13 @@
 #define LinkRelAttribute_h
 
 #include "core/dom/IconURL.h"
-
-namespace WTF {
-class String;
-}
+#include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 class LinkRelAttribute {
 public:
-    LinkRelAttribute();
-    explicit LinkRelAttribute(const String&);
+    explicit LinkRelAttribute(const String& = "");
 
     bool isStyleSheet() const { return m_isStyleSheet; }
     IconType iconType() const { return m_iconType; }