Refactor StyleVariableData to fallback on root map for performance reasons.

third_party/WebKit/Source/core/css/CSSComputedStyleDeclaration.h

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2008, 2012 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 47 matching lines @@
     void deref() override;
 #endif
 
     String getPropertyValue(CSSPropertyID) const;
     bool getPropertyPriority(CSSPropertyID) const;
 
     MutableStylePropertySet* copyProperties() const;
 
     CSSValue* getPropertyCSSValue(CSSPropertyID) const;
     CSSValue* getPropertyCSSValue(AtomicString customPropertyName) const;
-    const HashMap<AtomicString, RefPtr<CSSVariableData>>* getVariables() const;
+    std::unique_ptr<HashMap<AtomicString, RefPtr<CSSVariableData>>> getVariables() const;
 
     CSSValue* getFontSizeCSSValuePreferringKeyword() const;
     bool isMonospaceFont() const;
 
     MutableStylePropertySet* copyPropertiesInSet(const Vector<CSSPropertyID>&) const;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     CSSComputedStyleDeclaration(Node*, bool allowVisitedStyle, const String&);
@@ skipping 30 matching lines @@
     PseudoId m_pseudoElementSpecifier;
     bool m_allowVisitedStyle;
 #if !ENABLE(OILPAN)
     unsigned m_refCount;
 #endif
 };
 
 } // namespace blink
 
 #endif // CSSComputedStyleDeclaration_h