Cleanup ResourceLoadPriority setting

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 c4e60ef6cb0287542634fca608518185c3e21a0c..a62cb6a937df12a7209905da2f0158061672c426 100644
--- a/Source/core/fetch/ResourceFetcher.cpp
+++ b/Source/core/fetch/ResourceFetcher.cpp
@@ -57,78 +57,48 @@ using blink::WebURLRequest;
 
 namespace blink {
 
-ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const FetchRequest& request)
+static ResourceLoadPriority typeToPriority(Resource::Type type)
 {
-    // TODO(yoav): Change it here so that priority can be changed even after it was resolved.
-    if (request.priority() != ResourceLoadPriorityUnresolved)
-        return request.priority();
-
-    // An image fetch is used to distinguish between "early" and "late" scripts in a document
-    if (type == Resource::Image)
-        m_imageFetched = true;
-
-    // Runtime experiment that change how we prioritize resources.
-    // The toggles do not depend on each other and can be flipped individually
-    // though the cumulative result will depend on the interaction between them.
-    // Background doc: https://docs.google.com/document/d/1bCDuq9H1ih9iNjgzyAL0gpwNFiEP4TZS-YLRp_RuMlc/edit?usp=sharing
-    bool deferLateScripts = context().fetchDeferLateScripts();
-    bool increaseFontPriority = context().fetchIncreaseFontPriority();
-    bool increaseAsyncScriptPriority = context().fetchIncreaseAsyncScriptPriority();
-    // Increases the priorities for CSS, Scripts, Fonts and Images all by one level
-    // and parser-blocking scripts and visible images by 2.
-    // This is used in conjunction with logic on the Chrome side to raise the threshold
-    // of "layout-blocking" resources and provide a boost to resources that are needed
-    // as soon as possible for something currently on the screen.
-    bool increasePriorities = context().fetchIncreasePriorities();
-
     switch (type) {
     case Resource::MainResource:
-        return context().isLowPriorityIframe() ? ResourceLoadPriorityVeryLow : ResourceLoadPriorityVeryHigh;
+        return ResourceLoadPriorityVeryHigh;
+    case Resource::XSLStyleSheet:
+        ASSERT(RuntimeEnabledFeatures::xsltEnabled());
     case Resource::CSSStyleSheet:
-        return increasePriorities ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityHigh;
+        return ResourceLoadPriorityHigh;
     case Resource::Raw:
-        return request.options().synchronousPolicy == RequestSynchronously ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityMedium;
     case Resource::Script:
-        // Async/Defer scripts.
-        if (FetchRequest::LazyLoad == request.defer())
-            return increaseAsyncScriptPriority ? ResourceLoadPriorityMedium : ResourceLoadPriorityLow;
-        // Reduce the priority of late-body scripts.
-        if (deferLateScripts && request.forPreload() && m_imageFetched)
-            return increasePriorities ? ResourceLoadPriorityMedium : ResourceLoadPriorityLow;
-        // Parser-blocking scripts.
-        if (!request.forPreload())
-            return increasePriorities ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityMedium;
-        // Non-async/defer scripts discovered by the preload scanner (only early scripts if deferLateScripts is active).
-        return increasePriorities ? ResourceLoadPriorityHigh : ResourceLoadPriorityMedium;
     case Resource::Font:
-        if (increaseFontPriority)
-            return increasePriorities ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityHigh;
-        return increasePriorities ? ResourceLoadPriorityHigh : ResourceLoadPriorityMedium;
     case Resource::ImportResource:
         return ResourceLoadPriorityMedium;
-    case Resource::Image:
-        // Default images to VeryLow, and promote when they become visible.
-        return increasePriorities ? ResourceLoadPriorityLow : ResourceLoadPriorityVeryLow;
+    case Resource::LinkSubresource:
+    case Resource::TextTrack:
     case Resource::Media:
-        return ResourceLoadPriorityLow;
-    case Resource::XSLStyleSheet:
-        ASSERT(RuntimeEnabledFeatures::xsltEnabled());
-        return ResourceLoadPriorityHigh;
     case Resource::SVGDocument:
         return ResourceLoadPriorityLow;
+    case Resource::Image:
     case Resource::LinkPrefetch:
     case Resource::LinkPreload:
         return ResourceLoadPriorityVeryLow;
-    case Resource::LinkSubresource:
-        return ResourceLoadPriorityLow;
-    case Resource::TextTrack:
-        return ResourceLoadPriorityLow;
     }
 
     ASSERT_NOT_REACHED();
     return ResourceLoadPriorityUnresolved;
 }
 
+ResourceLoadPriority ResourceFetcher::loadPriority(Resource::Type type, const FetchRequest& request)
+{
+    // TODO(yoav): Change it here so that priority can be changed even after it was resolved.
+    if (request.priority() != ResourceLoadPriorityUnresolved)
+        return request.priority();
+
+    // Synchronous requests should always be max priority, lest they hang the renderer.
+    if (request.options().synchronousPolicy == RequestSynchronously)
+        return ResourceLoadPriorityHighest;
+
+    return context().modifyPriorityForExperiments(typeToPriority(type), type, request);
+}
+
 static void populateResourceTiming(ResourceTimingInfo* info, Resource* resource, bool clearLoadTimings)
 {
     info->setInitialRequest(resource->resourceRequest());
@@ -189,7 +159,6 @@ ResourceFetcher::ResourceFetcher(FetchContext* context)
     , m_autoLoadImages(true)
     , m_imagesEnabled(true)
     , m_allowStaleResources(false)
-    , m_imageFetched(false)
 {
 #if ENABLE(OILPAN)
     ThreadState::current()->registerPreFinalizer(this);