[css-ui] text-overflow and text-orientation: upright

Hi,

The spec says that U+2026 should be used for rendering ellipsis, however,
it seems to be a bit unfortunate when it is used with vertical text and
text-orientation is upright.

It causes something like

a
b
c
…

instead of

a
b
c
⋮

Not sure what's the best way to resolve this issue. Probably the spec could
state that, U+FE19 should be used in that specific case.

- Xidorn