VDom.getViewBox: handle the case of docEl:contain=paint
VDom.getViewBox: handle the case of docEl:contain=paint

In my tests on Chrome 55, the `contain` style is able to
make the full screen mode look like a whole mess.
So I have to give up code's correctness.

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus