Remove useless null checks from Node::document() call sites

Source/core/accessibility/AccessibilityObject.cpp

 /*
  * Copyright (C) 2008, 2009, 2011 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 487 matching lines @@
     }
 
     return parent;
 }
 
 AccessibilityObject* AccessibilityObject::firstAccessibleObjectFromNode(const Node* node)
 {
     if (!node)
         return 0;
 
-    Document* document = node->document();
-    if (!document)
-        return 0;
-
-    AXObjectCache* cache = document->axObjectCache();
-
+    AXObjectCache* cache = node->document()->axObjectCache();
     AccessibilityObject* accessibleObject = cache->getOrCreate(node->renderer());
     while (accessibleObject && accessibleObject->accessibilityIsIgnored()) {
         node = NodeTraversal::next(node);
 
         while (node && !node->renderer())
             node = NodeTraversal::nextSkippingChildren(node);
 
         if (!node)
             return 0;
 
@@ skipping 463 matching lines @@
 
     for (AccessibilityObject* object = parentObject(); object; object = object->parentObject()) {
         if (equalIgnoringCase(object->getAttribute(aria_hiddenAttr), "true"))
             return true;
     }
 
     return false;
 }
 
 } // namespace WebCore