Don't cache matched properties for elements without a flat-tree parent.

third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 95 matching lines @@
 {
     Vector<unsigned, 16> toRemove;
     for (const auto& cacheEntry : m_cache) {
         CachedMatchedProperties* cacheItem = cacheEntry.value.get();
         if (cacheItem->computedStyle->hasViewportUnits())
             toRemove.append(cacheEntry.key);
     }
     m_cache.removeAll(toRemove);
 }
 
-bool MatchedPropertiesCache::isCacheable(const ComputedStyle& style, const ComputedStyle& parentStyle)
+bool MatchedPropertiesCache::isCacheable(const ComputedStyle& style, const ComputedStyle& parentStyle, const ContainerNode* flatTreeParent)
 {
     if (style.unique() || (style.styleType() != PseudoIdNone && parentStyle.unique()))
         return false;
     if (style.zoom() != ComputedStyle::initialZoom())
         return false;
     if (style.getWritingMode() != ComputedStyle::initialWritingMode() || style.direction() != ComputedStyle::initialDirection())
         return false;
     // The cache assumes static knowledge about which properties are inherited.
-    if (parentStyle.hasExplicitlyInheritedProperties())
+    // Without a flat tree parent, StyleBuilder::applyProperty will not
+    // setHasExplicitlyInheritedProperties on the parent style.
+    if (!flatTreeParent || parentStyle.hasExplicitlyInheritedProperties())
         return false;
     if (style.hasVariableReferenceFromNonInheritedProperty())
         return false;
     return true;
 }
 
 DEFINE_TRACE(MatchedPropertiesCache)
 {
     visitor->trace(m_cache);
 }
 
 } // namespace blink