Enable `as` based priorities for <link rel=preload>

Source/core/loader/LinkLoader.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 127 matching lines @@
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, DebugMessageLevel, String("Preconnect triggered for " + href.host())));
             if (crossOrigin != CrossOriginAttributeNotSet) {
                 document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, DebugMessageLevel,
                     String("Preconnect CORS setting is ") + String((crossOrigin == CrossOriginAttributeAnonymous) ? "anonymous" : "use-credentials")));
             }
         }
         preconnect(href, crossOrigin);
     }
 }
 
-// TODO(yoav): Replace Resource:Type here with some way that conveys priority but not context.
-static bool getTypeFromAsAttribute(const String& as, Resource::Type& type)
+static bool getPriorityTypeFromAsAttribute(const String& as, Resource::Type& type)
 {
     if (as.isEmpty())
         return false;
-    // TODO(yoav): Return false also when the `as` value is not a valid one.
-    // TODO(yoav): Add actual types here and make sure priorities work accordingly.
-    type = Resource::LinkSubresource;
+
+    if (equalIgnoringCase(as, "image"))
+        type = Resource::Image;
+    else if (equalIgnoringCase(as, "script"))
+        type = Resource::Script;
+    else if (equalIgnoringCase(as, "stylesheet"))
+        type = Resource::CSSStyleSheet;
+    else
+        return false;
+
     return true;
 }
 
 void LinkLoader::preloadIfNeeded(const LinkRelAttribute& relAttribute, const KURL& href, Document& document, const String& as)
 {
     if (relAttribute.isLinkPreload()) {
         ASSERT(RuntimeEnabledFeatures::linkPreloadEnabled());
         if (!href.isValid() || href.isEmpty()) {
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, WarningMessageLevel, String("<link rel=preload> has an invalid `href` value")));
             return;
         }
-        Resource::Type type;
-        if (!getTypeFromAsAttribute(as, type)) {
+        // TODO(yoav): Figure out a way that 'as' would be used to set request headers.
+        Resource::Type priorityType;
+        if (!getPriorityTypeFromAsAttribute(as, priorityType)) {
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, WarningMessageLevel, String("<link rel=preload> must have a valid `as` value")));
             return;
         }
         FetchRequest linkRequest(ResourceRequest(document.completeURL(href)), FetchInitiatorTypeNames::link);
+        linkRequest.setPriority(ResourceFetcher::loadPriority(priorityType, linkRequest));
         Settings* settings = document.settings();
         if (settings && settings->logPreload())
             document.addConsoleMessage(ConsoleMessage::create(OtherMessageSource, DebugMessageLevel, String("Preload triggered for " + href.host() + href.path())));
-        setResource(document.fetcher()->fetchLinkPreloadResource(type, linkRequest));
+        setResource(document.fetcher()->fetchLinkPreloadResource(Resource::LinkPreload, linkRequest));
     }
 }
 
 bool LinkLoader::loadLinkFromHeader(const String& headerValue, Document* document)
 {
     if (!document)
         return false;
     LinkHeaderSet headerSet(headerValue);
     for (auto& header : headerSet) {
         if (!header.valid() || header.url().isEmpty() || header.rel().isEmpty())
             return false;
         LinkRelAttribute relAttribute(header.rel());
         KURL url = document->completeURL(header.url());
         if (RuntimeEnabledFeatures::linkHeaderEnabled())
             dnsPrefetchIfNeeded(relAttribute, url, *document);
 
         if (RuntimeEnabledFeatures::linkPreconnectEnabled())
             preconnectIfNeeded(relAttribute, url, *document, header.crossOrigin());
 
         // FIXME: Add more supported headers as needed.
     }
     return true;
 }
 
 bool LinkLoader::loadLink(const LinkRelAttribute& relAttribute, const AtomicString& crossOriginMode, const String& type, const String& as, const KURL& href, Document& document)
 {
+    // TODO(yoav): Do all links need to load only after they're in document???
+
     // TODO(yoav): Convert all uses of the CrossOriginAttribute to CrossOriginAttributeValue. crbug.com/486689
     // FIXME(crbug.com/463266): We're ignoring type here. Maybe we shouldn't.
     dnsPrefetchIfNeeded(relAttribute, href, document);
 
     preconnectIfNeeded(relAttribute, href, document, crossOriginAttributeValue(crossOriginMode));
 
-    preloadIfNeeded(relAttribute, href, document, as);
+    if (m_client->shouldLoadLink())
+        preloadIfNeeded(relAttribute, href, document, as);
 
     // FIXME(crbug.com/323096): Should take care of import.
     if ((relAttribute.isLinkPrefetch() || relAttribute.isLinkSubresource() || relAttribute.isTransitionExitingStylesheet()) && href.isValid() && document.frame()) {
         if (!m_client->shouldLoadLink())
             return false;
         Resource::Type type = relAttribute.isLinkSubresource() ?  Resource::LinkSubresource : Resource::LinkPrefetch;
         FetchRequest linkRequest(ResourceRequest(document.completeURL(href)), FetchInitiatorTypeNames::link);
         if (!crossOriginMode.isNull())
             linkRequest.setCrossOriginAccessControl(document.securityOrigin(), crossOriginMode);
         setResource(document.fetcher()->fetchLinkResource(type, linkRequest));
@@ skipping 23 matching lines @@
         m_prerender.clear();
     }
 }
 
 DEFINE_TRACE(LinkLoader)
 {
     visitor->trace(m_prerender);
 }
 
 }