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

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

Index: third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp b/third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp
index 3c86ee5319e15af2cd90d07ca020db16d4aa610b..072e2e98e815bb5ea40a3ead768904b076434e7b 100644
--- a/third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/MatchedPropertiesCache.cpp
@@ -113,7 +113,7 @@ void MatchedPropertiesCache::clearViewportDependent()
     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;
@@ -122,7 +122,9 @@ bool MatchedPropertiesCache::isCacheable(const ComputedStyle& style, const Compu
     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;