casper-bottom:fixed if always true in 26auto-deepin-installer

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/610720559e4b48adb36e817c3d2bde49/#70fe77f765fc4e83adbbce190a7caca9

Change-Id: I33e1566e6898684add8f967e493c1bbffa8395c4