Don't trigger full active style update on styleSheets access.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
@@ skipping 267 matching lines @@
   // CSSFontSelectorClient implementation.
   void FontsNeedUpdate(CSSFontSelector*) override;
 
  private:
   StyleEngine(Document&);
   bool NeedsActiveStyleSheetUpdate() const {
     return all_tree_scopes_dirty_ || tree_scopes_removed_ ||
            document_scope_dirty_ || dirty_tree_scopes_.size();
   }
 
-  TreeScopeStyleSheetCollection* EnsureStyleSheetCollectionFor(TreeScope&);
+  TreeScopeStyleSheetCollection& EnsureStyleSheetCollectionFor(TreeScope&);
   TreeScopeStyleSheetCollection* StyleSheetCollectionFor(TreeScope&);
   bool ShouldUpdateDocumentStyleSheetCollection() const;
   bool ShouldUpdateShadowTreeStyleSheetCollection() const;
 
   void MarkDocumentDirty();
   void MarkTreeScopeDirty(TreeScope&);
 
   bool IsMaster() const { return is_master_; }
   Document* Master();
   Document& GetDocument() const { return *document_; }
@@ skipping 98 matching lines @@
 
   std::unique_ptr<StyleResolverStats> style_resolver_stats_;
   unsigned style_for_element_count_ = 0;
 
   friend class StyleEngineTest;
 };
 
 }  // namespace blink
 
 #endif