Add Blink support for showing image placeholders using range requests.

third_party/WebKit/Source/core/loader/DocumentLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  * 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 145 matching lines @@
 void DocumentLoader::setSubresourceFilter(
     std::unique_ptr<WebDocumentSubresourceFilter> subresourceFilter) {
   m_subresourceFilter = std::move(subresourceFilter);
 }
 
 Resource* DocumentLoader::startPreload(Resource::Type type,
                                        FetchRequest& request) {
   Resource* resource = nullptr;
   switch (type) {
     case Resource::Image:
+      request.setPlaceholderImageRequestType(
+          m_frame && m_frame->settings() &&
+                  m_frame->settings()->fetchImagePlaceholders()
+              ? FetchRequest::AllowPlaceholder
+              : FetchRequest::DisallowPlaceholder);
+
       resource = ImageResource::fetch(request, fetcher());
       break;
     case Resource::Script:
       resource = ScriptResource::fetch(request, fetcher());
       break;
     case Resource::CSSStyleSheet:
       resource = CSSStyleSheetResource::fetch(request, fetcher());
       break;
     case Resource::Font:
       resource = FontResource::fetch(request, fetcher());
@@ skipping 585 matching lines @@
                              m_writer ? m_writer->encoding() : emptyAtom, true,
                              ForceSynchronousParsing);
   if (!source.isNull())
     m_writer->appendReplacingData(source);
   endWriting(m_writer.get());
 }
 
 DEFINE_WEAK_IDENTIFIER_MAP(DocumentLoader);
 
 }  // namespace blink