nsIPresShell *presShell = GetOwnerDoc()->GetShellAt(0);
nsIPresShell *presShell = doc->GetShellAt(0);
nsSVGSVGElement::GetPixelUnitToMillimeterX(float *aPixelUnitToMillimeterX){  // to correctly determine this, the caller would need to pass in the  // right PresContext...  *aPixelUnitToMillimeterX = 0.28f; // 90dpi  if (!IsInDoc()) return NS_OK;  // Get Presentation shell 0  nsIPresShell *presShell = GetOwnerDoc()->GetShellAt(0);  if (!presShell) return NS_OK;    // Get the Presentation Context from the Shell  nsPresContext *context = presShell->GetPresContext();  if (!context) return NS_OK;  *aPixelUnitToMillimeterX = context->ScaledPixelsToTwips() / TWIPS_PER_POINT_FLOAT / (72.0f * 0.03937f);  return NS_OK;}