Add toSVGRootInlineBox(), and use it as much as we can

Source/core/rendering/svg/SVGRootInlineBox.h

Index: Source/core/rendering/svg/SVGRootInlineBox.h
diff --git a/Source/core/rendering/svg/SVGRootInlineBox.h b/Source/core/rendering/svg/SVGRootInlineBox.h
index e4bd6ea05e7ab1c3918204579d71d94d6b65434d..41475cd2f3d852de7ac3d56fd7e16679c7fb4b23 100644
--- a/Source/core/rendering/svg/SVGRootInlineBox.h
+++ b/Source/core/rendering/svg/SVGRootInlineBox.h
@@ -62,6 +62,12 @@ private:
     float m_logicalHeight;
 };
 
+inline SVGRootInlineBox* toSVGRootInlineBox(RootInlineBox* box)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!box || box->isSVGRootInlineBox());
+    return static_cast<SVGRootInlineBox*>(box);
+}
+
 } // namespace WebCore
 
 #endif // SVGRootInlineBox_h