Don't cache nth-indices of pseudo elements.

Source/core/dom/NthIndexCache.cpp

Index: Source/core/dom/NthIndexCache.cpp
diff --git a/Source/core/dom/NthIndexCache.cpp b/Source/core/dom/NthIndexCache.cpp
index 01e721902a361ff88abae6757071ce9300d8dd05..498abafe26eae8179c1a6a9ad3ff00c5080761ad 100644
--- a/Source/core/dom/NthIndexCache.cpp
+++ b/Source/core/dom/NthIndexCache.cpp
@@ -37,6 +37,7 @@ NthIndexCache::NthIndexData& NthIndexCache::ensureNthIndexDataFor(Node& parent)
 
 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.