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

Source/core/svg/SVGUseElement.cpp

Index: Source/core/svg/SVGUseElement.cpp
diff --git a/Source/core/svg/SVGUseElement.cpp b/Source/core/svg/SVGUseElement.cpp
index 3ff826babec787ae574c67827c586484f61a4e86..90fcdc7690944fa5d366cb8294a0fa71161b5770 100644
--- a/Source/core/svg/SVGUseElement.cpp
+++ b/Source/core/svg/SVGUseElement.cpp
@@ -391,6 +391,10 @@ void SVGUseElement::buildShadowAndInstanceTree(SVGElement* target)
         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;