Add initial link preload support

Source/core/loader/LinkLoader.h

Index: Source/core/loader/LinkLoader.h
diff --git a/Source/core/loader/LinkLoader.h b/Source/core/loader/LinkLoader.h
index 264c018319a3140a63154b934d7848605343f65a..e9156f97d34b53a6217e11dab74185e2eb5ea657 100644
--- a/Source/core/loader/LinkLoader.h
+++ b/Source/core/loader/LinkLoader.h
@@ -63,7 +63,7 @@ public:
     virtual void didSendDOMContentLoadedForPrerender() override;
 
     void released();
-    bool loadLink(const LinkRelAttribute&, const AtomicString& crossOriginMode, const String& type, const KURL&, Document&);
+    bool loadLink(const LinkRelAttribute&, const AtomicString& crossOriginMode, const String& type, const String& as, const KURL&, Document&);
     static bool loadLinkFromHeader(const String& headerValue, Document*);
 
     DECLARE_TRACE();
@@ -71,6 +71,7 @@ public:
 private:
     void linkLoadTimerFired(Timer<LinkLoader>*);
     void linkLoadingErrorTimerFired(Timer<LinkLoader>*);
+    void preloadIfNeeded(const LinkRelAttribute&, const KURL& href, Document&, const String& as);
 
     LinkLoaderClient* m_client;