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

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

Index: third_party/WebKit/Source/core/dom/SelectorQuery.h
diff --git a/third_party/WebKit/Source/core/dom/SelectorQuery.h b/third_party/WebKit/Source/core/dom/SelectorQuery.h
index 3dbe765305c91073c0455ff50f81b27cc9a8c473..149cf5b386723a783fbf080fec4b8b15de7e0cb6 100644
--- a/third_party/WebKit/Source/core/dom/SelectorQuery.h
+++ b/third_party/WebKit/Source/core/dom/SelectorQuery.h
@@ -104,6 +104,10 @@ class CORE_EXPORT SelectorQuery {
                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;