Add Blink support for showing image placeholders using range requests.

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

 /*
  * Copyright (C) 2007 Apple 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 123 matching lines @@
   if (!m_frame->isLoading())
     return;
   // All of the progress bar completion policies besides LoadEvent instead block
   // on parsing completion, which corresponds to finishing parsing. For those
   // policies, don't consider resource load that start after DOMContentLoaded
   // finishes.
   if (m_frame->settings()->progressBarCompletion() !=
           ProgressBarCompletion::LoadEvent &&
       (m_finishedParsing || priority < ResourceLoadPriorityHigh))
     return;
-  DCHECK(!m_progressItems.get(identifier));
   m_progressItems.set(
       identifier,
       wrapUnique(new ProgressItem(progressItemDefaultEstimatedLength)));
 }
 
 void ProgressTracker::incrementProgress(unsigned long identifier,
                                         const ResourceResponse& response) {
   ProgressItem* item = m_progressItems.get(identifier);
   if (!item)
     return;
@@ skipping 76 matching lines @@
 void ProgressTracker::completeProgress(unsigned long identifier) {
   ProgressItem* item = m_progressItems.get(identifier);
   if (!item)
     return;
 
   item->estimatedLength = item->bytesReceived;
   maybeSendProgress();
 }
 
 }  // namespace blink