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

Source/core/css/CSSToLengthConversionData.h

Index: Source/core/css/CSSToLengthConversionData.h
diff --git a/Source/core/css/CSSToLengthConversionData.h b/Source/core/css/CSSToLengthConversionData.h
index f110b23d5da744179957a060f7d69f179df17811..6b81ce993b90d3a58beb9a423fd9005b184c4e37 100644
--- a/Source/core/css/CSSToLengthConversionData.h
+++ b/Source/core/css/CSSToLengthConversionData.h
@@ -46,7 +46,7 @@ public:
     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; }