Raise the loading priority of in-viewport images.

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 104 matching lines @@
         return ResourceLoadPriorityVeryHigh;
     case Resource::CSSStyleSheet:
         return ResourceLoadPriorityHigh;
     case Resource::Raw:
         return request.options().synchronousPolicy == RequestSynchronously ? ResourceLoadPriorityVeryHigh : ResourceLoadPriorityMedium;
     case Resource::Script:
     case Resource::Font:
     case Resource::ImportResource:
         return ResourceLoadPriorityMedium;
     case Resource::Image:
-        return request.forPreload() ? ResourceLoadPriorityVeryLow : ResourceLoadPriorityLow;
+        // 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::XSLStyleSheet:
         ASSERT(RuntimeEnabledFeatures::xsltEnabled());
         return ResourceLoadPriorityHigh;
     case Resource::SVGDocument:
         return ResourceLoadPriorityLow;
     case Resource::LinkPrefetch:
         return ResourceLoadPriorityVeryLow;
     case Resource::LinkSubresource:
         return ResourceLoadPriorityLow;
     case Resource::TextTrack:
@@ skipping 1185 matching lines @@
 }
 #endif
 
 const ResourceLoaderOptions& ResourceFetcher::defaultResourceOptions()
 {
     DEFINE_STATIC_LOCAL(ResourceLoaderOptions, options, (SendCallbacks, SniffContent, BufferData, AllowStoredCredentials, ClientRequestedCredentials, AskClientForCrossOriginCredentials, DoSecurityCheck, CheckContentSecurityPolicy, UseDefaultOriginRestrictionsForType, DocumentContext));
     return options;
 }
 
 }