Simplify the SelectorQuery constructor and reuse CSSSelectorList::length().

third_party/WebKit/Source/core/dom/SelectorQuery.h

 /*
  * Copyright (C) 2011, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Samsung Electronics. 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.
@@ skipping 86 matching lines @@
                    typename SelectorQueryTrait::OutputType&) const;
   template <typename SelectorQueryTrait>
   void executeSlowTraversingShadowTree(
       ContainerNode& rootNode,
       typename SelectorQueryTrait::OutputType&) const;
   template <typename SelectorQueryTrait>
   void execute(ContainerNode& rootNode,
                typename SelectorQueryTrait::OutputType&) const;
 
   CSSSelectorList m_selectorList;
+  // Contains the list of CSSSelector's to match, but without ones that could
+  // never match like pseudo elements, div::before. This can be empty, while
+  // m_selectorList will never be empty as SelectorQueryCache::add would have
+  // thrown an exception.
   Vector<const CSSSelector*> m_selectors;
   bool m_usesDeepCombinatorOrShadowPseudo : 1;
   bool m_needsUpdatedDistribution : 1;
 };
 
 class SelectorQueryCache {
   USING_FAST_MALLOC(SelectorQueryCache);
 
  public:
   SelectorQuery* add(const AtomicString&, const Document&, ExceptionState&);
   void invalidate();
 
  private:
   HashMap<AtomicString, std::unique_ptr<SelectorQuery>> m_entries;
 };
 
 }  // namespace blink
 
 #endif