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

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

Index: third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
diff --git a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
index 2357ad74eb163a9073b0a2a55d3c5e32ed02e8c0..61ec4687e9b44bad3d8fadf3a5a0700d50aeec7a 100644
--- a/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
+++ b/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp
@@ -1533,7 +1533,7 @@ void StyleResolver::applyMatchedProperties(StyleResolverState& state, const Matc
     bool applyInheritedOnly = false;
     const CachedMatchedProperties* cachedMatchedProperties = cacheHash ? m_matchedPropertiesCache.find(cacheHash, state, matchResult.matchedProperties()) : nullptr;
 
-    if (cachedMatchedProperties && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle())) {
+    if (cachedMatchedProperties && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle(), state.parentNode())) {

[hayato, 2016/08/30 02:01:05]

Could you explain briefly what parentStyle means if flatTreeParent is null? It
is never used?

[esprehn, 2016/08/30 21:37:10]

Can we just pass State into isCacheable? Passing all these members is kind of
silly

         INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyCacheHit, 1);
         // We can build up the style by copying non-inherited properties from an earlier style object built using the same exact
         // style declarations. We then only need to apply the inherited properties, if any, as their values can depend on the
@@ -1627,7 +1627,7 @@ void StyleResolver::applyMatchedProperties(StyleResolverState& state, const Matc
 
     loadPendingResources(state);
 
-    if (!cachedMatchedProperties && cacheHash && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle())) {
+    if (!cachedMatchedProperties && cacheHash && MatchedPropertiesCache::isCacheable(*state.style(), *state.parentStyle(), state.parentNode())) {
         ASSERT(RuntimeEnabledFeatures::styleMatchedPropertiesCacheEnabled());
         INCREMENT_STYLE_STATS_COUNTER(document().styleEngine(), matchedPropertyCacheAdded, 1);
         m_matchedPropertiesCache.add(*state.style(), *state.parentStyle(), cacheHash, matchResult.matchedProperties());