Ensure we don't crash when asking for variables that don't exist from the computed style.

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 1338 matching lines @@
         pair->append(zoomAdjustedPixelValueForLength(coordinate.x(), style));
         pair->append(zoomAdjustedPixelValueForLength(coordinate.y(), style));
         list->append(pair);
     }
 
     return list.release();
 }
 
 PassRefPtrWillBeRawPtr<CSSValue> ComputedStyleCSSValueMapping::get(const AtomicString customPropertyName, const ComputedStyle& style)
 {
-    return CSSCustomPropertyDeclaration::create(customPropertyName, style.variables()->getVariable(customPropertyName));
+    CSSVariableData* data = style.variables()->getVariable(customPropertyName);
+    if (!data)
+        return nullptr;
+
+    return CSSCustomPropertyDeclaration::create(customPropertyName, data);
 }
 
 PassRefPtrWillBeRawPtr<CSSValue> ComputedStyleCSSValueMapping::get(CSSPropertyID propertyID, const ComputedStyle& style, const LayoutObject* layoutObject, Node* styledNode, bool allowVisitedStyle)
 {
     const SVGComputedStyle& svgStyle = style.svgStyle();
     propertyID = CSSProperty::resolveDirectionAwareProperty(propertyID, style.direction(), style.writingMode());
     switch (propertyID) {
     case CSSPropertyInvalid:
         return nullptr;
 
@@ skipping 1346 matching lines @@
     case CSSPropertyAll:
         return nullptr;
     default:
         break;
     }
     ASSERT_NOT_REACHED();
     return nullptr;
 }
 
 }