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

third_party/WebKit/Source/core/dom/StyleSheetCollection.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 47 matching lines @@
   }
   const HeapVector<TraceWrapperMember<StyleSheet>>&
   StyleSheetsForStyleSheetList() const {
     return style_sheets_for_style_sheet_list_;
   }
 
   void Swap(StyleSheetCollection&);
   void SwapSheetsForSheetList(HeapVector<Member<StyleSheet>>&);
   void AppendActiveStyleSheet(const ActiveStyleSheet&);
   void AppendSheetForList(StyleSheet*);
+  void MarkSheetListDirty() { sheet_list_dirty_ = true; }
 
   DECLARE_VIRTUAL_TRACE();
   DECLARE_VIRTUAL_TRACE_WRAPPERS();
 
   void Dispose();
 
  protected:
   StyleSheetCollection();
 
   HeapVector<TraceWrapperMember<StyleSheet>> style_sheets_for_style_sheet_list_;
   ActiveStyleSheetVector active_author_style_sheets_;
+  bool sheet_list_dirty_ = true;
 };
 
 }  // namespace blink
 
 #endif  // StyleSheetCollection_h