Refactor StyleVariableData to fallback on root map for performance reasons.

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  * Copyright (C) 2015 Google 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
@@ skipping 1357 matching lines @@
     if (!variables)
         return nullptr;
 
     CSSVariableData* data = variables->getVariable(customPropertyName);
     if (!data)
         return nullptr;
 
     return CSSCustomPropertyDeclaration::create(customPropertyName, data);
 }
 
-const HashMap<AtomicString, RefPtr<CSSVariableData>>* ComputedStyleCSSValueMapping::getVariables(const ComputedStyle& style)
+PassOwnPtr<HashMap<AtomicString, RefPtr<CSSVariableData>>> ComputedStyleCSSValueMapping::getVariables(const ComputedStyle& style)
 {
     StyleVariableData* variables = style.variables();
     if (variables)
         return variables->getVariables();
     return nullptr;
 }
 
 PassRefPtrWillBeRawPtr<CSSValue> ComputedStyleCSSValueMapping::get(CSSPropertyID propertyID, const ComputedStyle& style, const LayoutObject* layoutObject, Node* styledNode, bool allowVisitedStyle)
 {
     const SVGComputedStyle& svgStyle = style.svgStyle();
@@ skipping 1365 matching lines @@
     case CSSPropertyAll:
         return nullptr;
     default:
         break;
     }
     ASSERT_NOT_REACHED();
     return nullptr;
 }
 
 } // namespace blink