Allow the "img.src='';img.src='.." idiom to clear down images immediately.

Source/core/loader/ImageLoader.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2009, 2010 Apple Inc. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 335 matching lines @@
     if (m_pendingTask) {
         m_pendingTask->clearLoader();
         m_pendingTask.clear();
     }
 
     KURL url = imageSourceToKURL(imageSourceURL);
     if (shouldLoadImmediately(url)) {
         doUpdateFromElement(DoNotBypassMainWorldCSP, updateBehavior);
         return;
     }
+    // Allow the idiom "img.src=''; img.src='.." to clear down the image before 
+    // an asynchronous load completes.
+    if (imageSourceURL.isEmpty()) {
+        ImageResource* image = m_image.get();
+        if (image)
+            image->removeClient(this);
+        m_image = nullptr;
+    }
     enqueueImageLoadingMicroTask(updateBehavior);
 }
 
 KURL ImageLoader::imageSourceToKURL(AtomicString imageSourceURL) const
 {
     KURL url;
 
     // Don't load images for inactive documents. We don't want to slow down the
     // raw HTML parsing case by loading images we don't intend to display.
     Document& document = m_element->document();
@@ skipping 228 matching lines @@
 #endif
 }
 
 #if ENABLE(OILPAN)
 ImageLoader::ImageLoaderClientRemover::~ImageLoaderClientRemover()
 {
     m_loader.willRemoveClient(m_client);
 }
 #endif
 }