Don't cache matched properties for elements without a flat-tree parent.

third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 81 matching lines @@
     {
         ASSERT(m_cache.isEmpty());
     }
 
     const CachedMatchedProperties* find(unsigned hash, const StyleResolverState&, const MatchedPropertiesVector&);
     void add(const ComputedStyle&, const ComputedStyle& parentStyle, unsigned hash, const MatchedPropertiesVector&);
 
     void clear();
     void clearViewportDependent();
 
-    static bool isCacheable(const ComputedStyle&, const ComputedStyle& parentStyle);
+    static bool isCacheable(const StyleResolverState&);
 
     DECLARE_TRACE();
 
 private:
     using Cache = HeapHashMap<unsigned, Member<CachedMatchedProperties>, DefaultHash<unsigned>::Hash, HashTraits<unsigned>, CachedMatchedPropertiesHashTraits>;
     Cache m_cache;
 };
 
 } // namespace blink
 
 #endif