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

third_party/WebKit/Source/core/dom/StyleSheetCollection.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 26 matching lines @@
 
 void StyleSheetCollection::Dispose() {
   style_sheets_for_style_sheet_list_.clear();
   active_author_style_sheets_.clear();
 }
 
 void StyleSheetCollection::Swap(StyleSheetCollection& other) {
   ::blink::swap(style_sheets_for_style_sheet_list_,
                 other.style_sheets_for_style_sheet_list_, this, &other);
   active_author_style_sheets_.swap(other.active_author_style_sheets_);
+  sheet_list_dirty_ = false;
 }
 
 void StyleSheetCollection::SwapSheetsForSheetList(
     HeapVector<Member<StyleSheet>>& sheets) {
-  // Only called for collection of HTML Imports that never has active sheets.
-  DCHECK(active_author_style_sheets_.IsEmpty());
   ::blink::swap(style_sheets_for_style_sheet_list_, sheets, this);
+  sheet_list_dirty_ = false;
 }
 
 void StyleSheetCollection::AppendActiveStyleSheet(
     const ActiveStyleSheet& active_sheet) {
   active_author_style_sheets_.push_back(active_sheet);
 }
 
 void StyleSheetCollection::AppendSheetForList(StyleSheet* sheet) {
   style_sheets_for_style_sheet_list_.push_back(
       TraceWrapperMember<StyleSheet>(this, sheet));
 }
 
 DEFINE_TRACE(StyleSheetCollection) {
   visitor->Trace(active_author_style_sheets_);
   visitor->Trace(style_sheets_for_style_sheet_list_);
 }
 
 DEFINE_TRACE_WRAPPERS(StyleSheetCollection) {
   for (auto sheet : style_sheets_for_style_sheet_list_) {
     visitor->TraceWrappers(sheet);
   }
 }
 
 }  // namespace blink