[svg] Update the <use> shadowtree if a referenced element is removed.

Source/core/svg/SVGUseElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
  * Copyright (C) 2011 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 University of Szeged
  * Copyright (C) 2012 Renata Hodovan <reni@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 373 matching lines @@
 
     // Clone the target subtree into the shadow tree, not handling <use> and <symbol> yet.
 
     // SVG specification does not say a word about <use> & cycles. My view on this is: just ignore it!
     // Non-appearing <use> content is easier to debug, then half-appearing content.
     if (!buildShadowTree(target, m_targetElementInstance.get(), false)) {
         clearResourceReferences();
         return;
     }
 
+    // Add a reference such that if the target is removed we tear down the
+    // corresponding shadowtree.
+    addReferenceTo(target);

[pdr., 2015/04/17 04:57:25]

It's been a while since I've been in SVGUseElement so please forgive some paging
out that occurred.

IIRC, we have two element "map" concepts:
1) the ingoing/outgoing reference machinery used in addReferenceTo.
2) the instances of use elements (instancesForElement, invalidateInstances, etc)

I would have expected this to have been handled through the existing <use>
element's SVGElement::removedFrom which calls invalidateInstances. Do you know
why it isn't?

+
     if (instanceTreeIsLoading(m_targetElementInstance.get()))
         return;
 
     // Assure shadow tree building was successfull
     ASSERT(m_targetElementInstance);
     ASSERT(m_targetElementInstance->correspondingUseElement() == this);
     ASSERT(m_targetElementInstance->correspondingElement() == target);
 
     // Expand all <use> elements in the shadow tree.
     // Expand means: replace the actual <use> element by what it references.
@@ skipping 358 matching lines @@
 
     if (m_resource)
         m_resource->removeClient(this);
 
     m_resource = resource;
     if (m_resource)
         m_resource->addClient(this);
 }
 
 }