Defer late and async scripts

Source/core/fetch/ResourceFetcher.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.com/
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
@@ skipping 101 matching lines @@
 }
 
 static ResourceLoadPriority loadPriority(Resource::Type type, const FetchRequest& request)
 {
     if (request.priority() != ResourceLoadPriorityUnresolved)
         return request.priority();
 
     switch (type) {
     case Resource::MainResource:
         return ResourceLoadPriorityVeryHigh;
-    case Resource::CSSStyleSheet:
-        return ResourceLoadPriorityHigh;
     case Resource::Raw:
         return request.options().synchronousPolicy == RequestSynchronously ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityMedium;
+    case Resource::CSSStyleSheet:
     case Resource::Script:
+        // CSS and Script get the same treatment and priority levels so we load them in order
+        // and they can block each other's parsing/execution (with the exception of async scripts).
+        // Async scripts do not block the parser so they get the lowest priority and can be
+        // loaded in parser order with images.
+        if (type == Resource::Script && request.execAsync())
+            return ResourceLoadPriorityVeryLow;
+        // Scripts or CSS discovered by the preload scanner that are not in the head get a low priority
+        // so they do not load while we are loading the layout-blocking resources but still load
+        // at a higher priority than images since they block the main parser.
+        if (request.forPreload() && !request.isBeforeBody())
+            return ResourceLoadPriorityLow;
+        return ResourceLoadPriorityMedium;
     case Resource::Font:
     case Resource::ImportResource:
         return ResourceLoadPriorityMedium;
     case Resource::Image:
         // We'll default images to VeryLow, and promote whatever is visible. This improves
         // speed-index by ~5% on average, ~14% at the 99th percentile.
         return ResourceLoadPriorityVeryLow;
     case Resource::Media:
         return ResourceLoadPriorityLow;
     case Resource::XSLStyleSheet:
@@ skipping 1403 matching lines @@
 
 void ResourceFetcher::trace(Visitor* visitor)
 {
     visitor->trace(m_document);
     visitor->trace(m_loaders);
     visitor->trace(m_multipartLoaders);
     ResourceLoaderHost::trace(visitor);
 }
 
 }