Defer late and async scripts

Source/core/fetch/ResourceFetcher.cpp

Index: Source/core/fetch/ResourceFetcher.cpp
diff --git a/Source/core/fetch/ResourceFetcher.cpp b/Source/core/fetch/ResourceFetcher.cpp
index 14b6167d7ac605ee120b0e6ec91fb4df30c8e04b..7353742b76fae1369c72b9d24af2c9352fd291cf 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -119,11 +119,22 @@ static ResourceLoadPriority loadPriority(Resource::Type type, const FetchRequest
     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;