void SetViewportOverflowOverride(PRUint8 aX, PRUint8 aY)
void SetViewportOverflowOverride(PRUint8 aOverflow)
  void SetViewportOverflowOverride(PRUint8 aX, PRUint8 aY)  {    mViewportStyleOverflow.mHorizontal = aX;    mViewportStyleOverflow.mVertical = aY;  }