Handle SVGLength resolving in an inactive document gracefully

Source/core/svg/SVGLengthContext.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 47 matching lines @@
     if (!context)
         return nullptr;
 
     const ContainerNode* currentContext = context;
     do {
         if (currentContext->layoutObject())
             return currentContext->layoutObject()->style();
         currentContext = currentContext->parentNode();
     } while (currentContext);
 
-    // There must be at least a LayoutSVGRoot layoutObject, carrying a style.
-    ASSERT_NOT_REACHED();
+    // We can end up here if trying to resolve values for elements in an
+    // inactive document.
     return nullptr;
 }
 
 static const ComputedStyle* rootElementStyle(const Node* context)
 {
     if (!context)
         return nullptr;
 
     const Document& document = context->document();
     Node* documentElement = document.documentElement();
@@ skipping 268 matching lines @@
 
     const SVGSVGElement& svg = toSVGSVGElement(*viewportElement);
     viewportSize = svg.currentViewBoxRect().size();
     if (viewportSize.isEmpty())
         viewportSize = svg.currentViewportSize();
 
     return true;
 }
 
 }