Don't cache nth-indices of pseudo elements.

Source/core/dom/NthIndexCache.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NthIndexCache_h
 #define NthIndexCache_h
 
 #include "core/CoreExport.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementTraversal.h"
@@ skipping 38 matching lines @@
 
     using ParentMap = WillBeHeapHashMap<RefPtrWillBeMember<Node>, OwnPtrWillBeMember<NthIndexData>>;
 
     OwnPtrWillBeMember<ParentMap> m_parentMap;
     RawPtrWillBeMember<Document> m_document;
     uint64_t m_domTreeVersion;
 };
 
 inline unsigned NthIndexCache::NthIndexData::nthIndex(Element& element)
 {
+    if (element.isPseudoElement())
+        return 1;
     if (!m_count)
         return cacheNthIndices(element);
 
     unsigned index = 0;
     for (Element* sibling = &element; sibling; sibling = ElementTraversal::previousSibling(*sibling), index++) {
         auto it = m_elementIndexMap.find(sibling);
         if (it != m_elementIndexMap.end())
             return it->value + index;
     }
     return index;
 }
 
 inline unsigned NthIndexCache::NthIndexData::nthLastIndex(Element& element)
 {
+    if (element.isPseudoElement())
+        return 1;
     unsigned index = nthIndex(element);
     return m_count - index + 1;
 }
 
 inline unsigned NthIndexCache::nthChildIndex(Element& element)
 {
     ASSERT(element.parentNode());
     return ensureNthIndexDataFor(*element.parentNode()).nthIndex(element);
 }
 
 inline unsigned NthIndexCache::nthLastChildIndex(Element& element)
 {
     ASSERT(element.parentNode());
     return ensureNthIndexDataFor(*element.parentNode()).nthLastIndex(element);
 }
 
 } // namespace blink
 
 #endif // NthIndexCache_h