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

third_party/WebKit/Source/core/dom/TreeScopeStyleSheetCollection.cpp

 /*
  * 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 61 matching lines @@
 
 bool TreeScopeStyleSheetCollection::HasStyleSheets() const {
   for (Node* node : style_sheet_candidate_nodes_) {
     StyleSheetCandidate candidate(*node);
     if (candidate.Sheet() || candidate.IsEnabledAndLoading())
       return true;
   }
   return false;
 }
 
-void TreeScopeStyleSheetCollection::CollectStyleSheetsForList() {
+void TreeScopeStyleSheetCollection::UpdateStyleSheetList() {
+  if (!sheet_list_dirty_)
+    return;
+
   HeapVector<Member<StyleSheet>> new_list;
   for (Node* node : style_sheet_candidate_nodes_) {
     StyleSheetCandidate candidate(*node);
     DCHECK(!candidate.IsXSL());
     if (candidate.IsImport())
       continue;
     if (candidate.IsEnabledAndLoading())
       continue;
     if (StyleSheet* sheet = candidate.Sheet())
       new_list.push_back(sheet);
   }
   SwapSheetsForSheetList(new_list);
 }
 
 DEFINE_TRACE(TreeScopeStyleSheetCollection) {
   visitor->Trace(tree_scope_);
   visitor->Trace(style_sheet_candidate_nodes_);
   StyleSheetCollection::Trace(visitor);
 }
 
 }  // namespace blink