Check for styleResolver() in preparation for async style update.

third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.cpp

 /*
  * 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) 2012 Google Inc. 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
@@ skipping 61 matching lines @@
 
   Document& document = treeScope().document();
   CSSFontSelector* cssFontSelector = document.styleEngine().fontSelector();
   const HeapVector<Member<StyleRuleFontFace>> fontFaceRules =
       ruleSet.fontFaceRules();
   for (auto& fontFaceRule : fontFaceRules) {
     if (FontFace* fontFace = FontFace::create(&document, fontFaceRule))
       cssFontSelector->fontFaceCache()->add(cssFontSelector, fontFaceRule,
                                             fontFace);
   }
-  if (fontFaceRules.size())
+  if (fontFaceRules.size() && document.styleResolver())
     document.styleResolver()->invalidateMatchedPropertiesCache();
 }
 
 void ScopedStyleResolver::appendCSSStyleSheet(
     CSSStyleSheet& cssSheet,
     const MediaQueryEvaluator& medium) {
   unsigned index = m_authorStyleSheets.size();
   m_authorStyleSheets.append(&cssSheet);
   StyleSheetContents* sheet = cssSheet.contents();
   AddRuleFlags addRuleFlags =
@@ skipping 211 matching lines @@
   m_treeBoundaryCrossingRuleSet->append(
       RuleSubSet::create(parentStyleSheet, sheetIndex, ruleSetForScope));
 }
 
 DEFINE_TRACE(ScopedStyleResolver::RuleSubSet) {
   visitor->trace(m_parentStyleSheet);
   visitor->trace(m_ruleSet);
 }
 
 }  // namespace blink