[css-ruby] Computing writing-mode for inter-character ruby

The current spec says:

This value forces the computed value of writing-mode of the ruby annotation
container to be vertical-rl.

And according to the discussions in CSSWG F2F meeting in Sydney, because of
the cyclic dependency between writing-mode and display, the spec should be
changed to something like

The writing-mode of a ruby annotation is computed to vertical-rl if the
ruby-position of its container is inter-character.

But both forms have a problem encountered from the inheriting rule of
anonymous boxes. Since either ruby annotation or its container could be an
anonymous box (but never both), in which case, its computed writing-mode
value cannot be inherited by its descendants.

I have no idea how to solve this. If the cyclic dependency above is
allowed, though, probably we can say the writing-mode is computed to
vertical-rl for both ruby annotation and ruby annotation container.

- Xidorn