Fixed ::slotted performance in pure v1 shadow documents.

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.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 709 matching lines @@
         return;
     }
     if (!m_styleResolverStats)
         m_styleResolverStats = StyleResolverStats::create();
     else
         m_styleResolverStats->reset();
 }
 
 void StyleEngine::setShadowCascadeOrder(ShadowCascadeOrder order)
 {
-    if (order <= m_shadowCascadeOrder)
+    ASSERT(order != ShadowCascadeOrder::ShadowCascadeNone);
+
+    if (order == m_shadowCascadeOrder)
         return;
 
+    if (order == ShadowCascadeOrder::ShadowCascadeV0)
+        m_mayContainV0Shadow = true;
+
     // For V0 -> V1 upgrade, we need style recalculation for the whole document.
     if (m_shadowCascadeOrder == ShadowCascadeOrder::ShadowCascadeV0 && order == ShadowCascadeOrder::ShadowCascadeV1)
         document().setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::Shadow));
 
-    m_shadowCascadeOrder = order;
+    if (order > m_shadowCascadeOrder)
+        m_shadowCascadeOrder = order;

[kochi, 2016/04/04 04:48:09]

Aha, good catch!
createShadowRoot() could set this to V0 from V1 (downgrade), which we don't
want.

 }
 
 void StyleEngine::setPreferredStylesheetSetNameIfNotSet(const String& name)
 {
     if (!m_preferredStylesheetSetName.isEmpty())
         return;
     m_preferredStylesheetSetName = name;
     // TODO(rune@opera.com): Setting the selected set here is wrong if the set
     // has been previously set by through Document.selectedStylesheetSet. Our
     // current implementation ignores the effect of Document.selectedStylesheetSet
@@ skipping 42 matching lines @@
     visitor->trace(m_dirtyTreeScopes);
     visitor->trace(m_activeTreeScopes);
     visitor->trace(m_fontSelector);
     visitor->trace(m_textToSheetCache);
     visitor->trace(m_sheetToTextCache);
 #endif
     CSSFontSelectorClient::trace(visitor);
 }
 
 } // namespace blink