Avoid always style recalc when removing stylesheets.

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.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 270 matching lines @@
     if (notification == RemovePendingSheetNotifyLater) {
         m_document.setNeedsNotifyRemoveAllPendingStylesheet();
         return;
     }
 
     // FIXME: We can't call addedStyleSheet or removedStyleSheet here because we don't know
     // what's new. We should track that to tell the style system what changed.
     m_document.didRemoveAllPendingStylesheet();
 }
 
+void StyleEngine::modifiedStyleSheet(StyleSheet* sheet)
+{
+    if (!sheet)
+        return;
+
+    Node* node = sheet->ownerNode();
+    if (!node || !node->inDocument())
+        return;
+
+    TreeScope& treeScope = isHTMLStyleElement(node) ? node->treeScope() : m_document;
+    ASSERT(isHTMLStyleElement(node) || treeScope == m_document);
+
+    if (treeScope == m_document) {
+        m_needsDocumentStyleSheetsUpdate = true;
+        return;
+    }
+    m_dirtyTreeScopes.add(&treeScope);
+}
+
 void StyleEngine::addStyleSheetCandidateNode(Node* node, bool createdByParser)
 {
     if (!node->inDocument())
         return;
 
     TreeScope& treeScope = isHTMLStyleElement(node) ? node->treeScope() : m_document;
     ASSERT(isHTMLStyleElement(node) || treeScope == m_document);
 
     StyleSheetCollection* collection = ensureStyleSheetCollectionFor(treeScope);
     ASSERT(collection);
@@ skipping 138 matching lines @@
         if (StyleSheetCollection* collection = m_styleSheetCollectionMap.get(*it)) {
             styleResolver->setBuildScopedStyleTreeInDocumentOrder(!collection->scopingNodesForStyleScoped());
             styleResolver->appendAuthorStyleSheets(0, collection->activeAuthorStyleSheets());
         }
     }
     styleResolver->finishAppendAuthorStyleSheets();
     styleResolver->setBuildScopedStyleTreeInDocumentOrder(false);
 }
 
 }