Attempt to fix issue 344300 by guarding against rootStyle being null.

Source/core/css/CSSToLengthConversionData.h

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 28 matching lines @@
 class RenderStyle;
 class RenderView;
 
 class CSSToLengthConversionData {
 public:
     CSSToLengthConversionData(const RenderStyle* currStyle, const RenderStyle* rootStyle, const RenderView*, float zoom, bool computingFontSize = false);
     CSSToLengthConversionData(const RenderStyle* currStyle, const RenderStyle* rootStyle, const RenderView*, bool computingFontSize = false);
     CSSToLengthConversionData(const RenderStyle* currStyle, const RenderStyle* rootStyle, float viewportWidth, float viewportHeight, float zoom, bool computingFontSize = false);
 
     const RenderStyle& style() const { return *m_style; }
-    const RenderStyle& rootStyle() const { return *m_rootStyle; }
+    const RenderStyle* rootStyle() const { return m_rootStyle; }
     float zoom() const;
     bool computingFontSize() const { return m_computingFontSize; }
 
     // Accessing these marks the style as having viewport units
     double viewportWidthPercent() const;
     double viewportHeightPercent() const;
     double viewportMinPercent() const;
     double viewportMaxPercent() const;
 
     void setStyle(const RenderStyle* style) { m_style = style; }
@@ skipping 10 matching lines @@
     float m_viewportWidth;
     float m_viewportHeight;
     float m_zoom;
     bool m_useEffectiveZoom;
     bool m_computingFontSize;
 };
 
 } // namespace WebCore
 
 #endif