Fixed ::slotted performance in pure v1 shadow documents.

third_party/WebKit/Source/core/dom/StyleEngine.cpp

Index: third_party/WebKit/Source/core/dom/StyleEngine.cpp
diff --git a/third_party/WebKit/Source/core/dom/StyleEngine.cpp b/third_party/WebKit/Source/core/dom/StyleEngine.cpp
index 6e10d0a669856f096c4619b06d57cf270511920e..986e787de3d78efb077c212d4ac39e35f5b01cdc 100644
--- a/third_party/WebKit/Source/core/dom/StyleEngine.cpp
+++ b/third_party/WebKit/Source/core/dom/StyleEngine.cpp
@@ -727,14 +727,20 @@ void StyleEngine::setStatsEnabled(bool enabled)
 
 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)