Don't cache nth-indices of pseudo elements.

Source/core/dom/NthIndexCache.cpp

 // 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.
 
 #include "config.h"
 #include "core/dom/NthIndexCache.h"
 
 #include "core/dom/Document.h"
 
 namespace blink {
@@ skipping 19 matching lines @@
     ParentMap::AddResult addResult = m_parentMap->add(&parent, nullptr);
     if (addResult.isNewEntry)
         addResult.storedValue->value = adoptPtrWillBeNoop(new NthIndexData());
 
     ASSERT(addResult.storedValue->value);
     return *addResult.storedValue->value;
 }
 
 unsigned NthIndexCache::NthIndexData::cacheNthIndices(Element& element)
 {
+    ASSERT(!element.isPseudoElement());
     unsigned index = 0;
     // The frequency at which we cache the nth-index for a set of siblings.
     // A spread value of 3 means every third Element will have its nth-index cached.
     // Using a spread value > 1 is done to save memory. Looking up the nth-index will
     // still be done in constant time in terms of sibling count, at most 'spread'
     // elements will be traversed.
     const unsigned spread = 3;
     unsigned count = 0;
     for (Element* sibling = ElementTraversal::firstChild(*element.parentNode()); sibling; sibling = ElementTraversal::nextSibling(*sibling)) {
         if (!(++count % spread))
@@ skipping 13 matching lines @@
 #endif
 }
 
 #if !ENABLE(OILPAN)
 NthIndexCache::NthIndexData::~NthIndexData()
 {
 }
 #endif
 
 } // namespace blink