Update styleSheets list in import without active style update.

third_party/WebKit/Source/core/dom/StyleEngine.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, 2011, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
@@ skipping 103 matching lines @@
 
   StyleSheetCollectionMap::iterator it =
       style_sheet_collection_map_.find(&tree_scope);
   if (it == style_sheet_collection_map_.end())
     return nullptr;
   return it->value.Get();
 }
 
 const HeapVector<TraceWrapperMember<StyleSheet>>&
 StyleEngine::StyleSheetsForStyleSheetList(TreeScope& tree_scope) {
-  // TODO(rune@opera.com): we could split styleSheets and active stylesheet
-  // update to have a lighter update while accessing the styleSheets list.
   DCHECK(Master());
   if (Master()->IsActive()) {
-    if (IsMaster())
+    if (IsMaster()) {
+      // TODO(rune@opera.com): Replace with UpdateStyleSheetList().
       UpdateActiveStyle();
-    else
-      Master()->GetStyleEngine().UpdateActiveStyle();
+    } else {
+      UpdateStyleSheetList(tree_scope);
+    }
   }
 
   if (tree_scope == document_)
     return GetDocumentStyleSheetCollection().StyleSheetsForStyleSheetList();
 
+  DCHECK(IsMaster());
   return EnsureStyleSheetCollectionFor(tree_scope)
       ->StyleSheetsForStyleSheetList();
 }
 
+void StyleEngine::UpdateStyleSheetList(TreeScope& tree_scope) {
+  // TODO(rune@opera.com): currently only for import documents.
+  DCHECK(!IsMaster());
+  DCHECK(tree_scope.GetDocument() == GetDocument());
+
+  if (tree_scope != GetDocument())
+    return;
+  if (!ShouldUpdateDocumentStyleSheetCollection())
+    return;
+
+  GetDocumentStyleSheetCollection().CollectStyleSheetsForList();
+  // Avoid updating the styleSheets list repeatedly. Only safe for imports.
+  document_scope_dirty_ = false;
+  DCHECK(!all_tree_scopes_dirty_);
+}
+
 void StyleEngine::InjectAuthorSheet(StyleSheetContents* author_sheet) {
   injected_author_style_sheets_.push_back(TraceWrapperMember<CSSStyleSheet>(
       this, CSSStyleSheet::Create(author_sheet, *document_)));
   MarkDocumentDirty();
 }
 
 CSSStyleSheet& StyleEngine::EnsureInspectorStyleSheet() {
   if (inspector_style_sheet_)
     return *inspector_style_sheet_;
 
@@ skipping 1057 matching lines @@
 }
 
 DEFINE_TRACE_WRAPPERS(StyleEngine) {
   for (auto sheet : injected_author_style_sheets_) {
     visitor->TraceWrappers(sheet);
   }
   visitor->TraceWrappers(document_style_sheet_collection_);
 }
 
 }  // namespace blink