Avoid unnecessary updateActiveStyle comparing shadow styles.

third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
@@ skipping 58 matching lines @@
   void collectFeaturesTo(RuleFeatureSet&,
                          HeapHashSet<Member<const StyleSheetContents>>&
                              visitedSharedStyleSheetContents) const;
   void resetAuthorStyle();
   bool hasDeepOrShadowSelector() const { return m_hasDeepOrShadowSelector; }
   void setHasUnresolvedKeyframesRule() { m_hasUnresolvedKeyframesRule = true; }
   bool needsAppendAllSheets() const { return m_needsAppendAllSheets; }
   void setNeedsAppendAllSheets() { m_needsAppendAllSheets = true; }
   static void keyframesRulesAdded(const TreeScope&);
   static ContainerNode& invalidationRootForTreeScope(const TreeScope&);
+  CORE_EXPORT static bool haveSameStyles(const ScopedStyleResolver*,

[sashab, 2017/01/04 00:39:33]

I sort of agree with sigbjornf that this could be a bool operator==, but it
sounds like you're not actually comparing equality, just that the two styles are
equal in the same order. Also either could be null.

+                                         const ScopedStyleResolver*);
 
   DECLARE_TRACE();
 
  private:
   explicit ScopedStyleResolver(TreeScope& scope) : m_scope(scope) {}
 
   void addTreeBoundaryCrossingRules(const RuleSet&,
                                     CSSStyleSheet*,
                                     unsigned sheetIndex);
   void addKeyframeRules(const RuleSet&);
@@ skipping 32 matching lines @@
 
   Member<CSSStyleSheetRuleSubSet> m_treeBoundaryCrossingRuleSet;
   bool m_hasDeepOrShadowSelector = false;
   bool m_hasUnresolvedKeyframesRule = false;
   bool m_needsAppendAllSheets = false;
 };
 
 }  // namespace blink
 
 #endif  // ScopedStyleResolver_h