Bug 1074165 - Prevent out of range scrolling in nsListboxBodyFrame.  r=mats
- nsListBoxBodyFrame::UpdateIndex now uses the position returned by
  nsScrollBarFrame::MoveToNewPosition to calculate the new row position.
- Code used to calculate the row position from the scroll position has been
  moved to a new function, ToRowIndex.
- nsListBoxBodyFrame::ThumbMoved has been updated to use ToRowIndex.