Add initial link preload support

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 f18a31e220ece361592f02ff327d6e191dcda7ae..ee2151c11ef36602a50581e4c20aa4935996c38b 100644
--- a/Source/core/html/LinkRelAttribute.h
+++ b/Source/core/html/LinkRelAttribute.h
@@ -48,6 +48,7 @@ public:
     bool isPreconnect() const { return m_isPreconnect; }
     bool isLinkPrefetch() const { return m_isLinkPrefetch; }
     bool isLinkSubresource() const { return m_isLinkSubresource; }
+    bool isLinkPreload() const { return m_isLinkPreload; }
     bool isLinkPrerender() const { return m_isLinkPrerender; }
     bool isLinkNext() const { return m_isLinkNext; }
     bool isImport() const { return m_isImport; }
@@ -63,6 +64,7 @@ private:
     bool m_isPreconnect : 1;
     bool m_isLinkPrefetch : 1;
     bool m_isLinkSubresource : 1;
+    bool m_isLinkPreload : 1;
     bool m_isLinkPrerender : 1;
     bool m_isLinkNext : 1;
     bool m_isImport : 1;