Avoid always style recalc when removing stylesheets.

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)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 59 matching lines @@
 void StyleSheetCollection::removeStyleSheetCandidateNode(Node* node, ContainerNode* scopingNode)
 {
     m_styleSheetCandidateNodes.remove(node);
 
     if (!isTreeScopeRoot(scopingNode))
         m_scopingNodesForStyleScoped.remove(scopingNode);
 }
 
 StyleSheetCollection::StyleResolverUpdateType StyleSheetCollection::compareStyleSheets(const Vector<RefPtr<CSSStyleSheet> >& oldStyleSheets, const Vector<RefPtr<CSSStyleSheet> >& newStylesheets, Vector<StyleSheetContents*>& addedSheets)
 {
-    // Find out which stylesheets are new.
     unsigned newStylesheetCount = newStylesheets.size();
     unsigned oldStylesheetCount = oldStyleSheets.size();
-    if (newStylesheetCount < oldStylesheetCount)
-        return Reconstruct;
+    ASSERT(newStylesheetCount >= oldStylesheetCount);
 
     unsigned newIndex = 0;
     for (unsigned oldIndex = 0; oldIndex < oldStylesheetCount; ++oldIndex) {
         if (newIndex >= newStylesheetCount)
             return Reconstruct;
         while (oldStyleSheets[oldIndex] != newStylesheets[newIndex]) {
             addedSheets.append(newStylesheets[newIndex]->contents());
             ++newIndex;
             if (newIndex == newStylesheetCount)
                 return Reconstruct;
@@ skipping 35 matching lines @@
     if (activeLoadingStyleSheetLoaded(newStyleSheets))
         return;
 
     if (updateMode != AnalyzedStyleUpdate)
         return;
     if (!document()->styleResolverIfExists())
         return;
 
     // Find out which stylesheets are new.
     Vector<StyleSheetContents*> addedSheets;
-    styleResolverUpdateType = compareStyleSheets(oldStyleSheets, newStyleSheets, addedSheets);
+    if (oldStyleSheets.size() <= newStyleSheets.size()) {
+        styleResolverUpdateType = compareStyleSheets(oldStyleSheets, newStyleSheets, addedSheets);
+    } else {
+        StyleResolverUpdateType updateType = compareStyleSheets(newStyleSheets, oldStyleSheets, addedSheets);
+        styleResolverUpdateType = updateType != Additive ? updateType : Reset;
+    }
 
     // FIXME: If styleResolverUpdateType is still Reconstruct, we could return early here
     // as destroying the StyleResolver will recalc the whole document anyway?
 
     // If we are already parsing the body and so may have significant amount of elements, put some effort into trying to avoid style recalcs.
     if (!document()->body() || document()->hasNodesWithPlaceholderStyle())
         return;
     StyleInvalidationAnalysis invalidationAnalysis(addedSheets);
     if (invalidationAnalysis.dirtiesAllStyle())
         return;
@@ skipping 23 matching lines @@
     }
     return false;
 }
 
 void StyleSheetCollection::updateUsesRemUnits()
 {
     m_usesRemUnits = styleSheetsUseRemUnits(m_activeAuthorStyleSheets);
 }
 
 }