Revert 146726 "AXObjectCache gets recreated during document tear..."

Source/WebCore/accessibility/AXObjectCache.cpp

 /*
  * Copyright (C) 2008, 2009, 2010 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
@@ skipping 828 matching lines @@
 }
 
 void AXObjectCache::startCachingComputedObjectAttributesUntilTreeMutates()
 {
     if (!m_computedObjectAttributeCache)
         m_computedObjectAttributeCache = AXComputedObjectAttributeCache::create();
 }
 
 void AXObjectCache::stopCachingComputedObjectAttributes()
 {
-    if (m_computedObjectAttributeCache)
-        m_computedObjectAttributeCache.clear();
+    m_computedObjectAttributeCache.clear();
 }
 
 VisiblePosition AXObjectCache::visiblePositionForTextMarkerData(TextMarkerData& textMarkerData)
 {
     if (!isNodeInUse(textMarkerData.node))
         return VisiblePosition();
     
     // FIXME: Accessability should make it clear these are DOM-compliant offsets or store Position objects.
     VisiblePosition visiblePos = VisiblePosition(createLegacyEditingPosition(textMarkerData.node, textMarkerData.offset), textMarkerData.affinity);
     Position deepPos = visiblePos.deepEquivalent();
@@ skipping 76 matching lines @@
     
     if (!node->isElementNode())
         return false;
     
     return equalIgnoringCase(toElement(node)->getAttribute(aria_hiddenAttr), "false");
 }
 
 } // namespace WebCore
 
 #endif // HAVE(ACCESSIBILITY)