Change how scrollShift is calculated.

    In Firefox $('body').scrollTop returns 0, that's why the modal
    appear outside the visible screen. Added $('hmtl).scrollTop.

    var scrollShift = $('body').scrollTop() || $('html').scrollTop()

    Closes-Bug: 1251361

Change-Id: Icf691eb140e091615a98f1fbd415428cd418ba9e
Merge "Change how scrollShift is calculated."