[css-ruby] Default font-size of ruby annotation with ruby-position: inter-character

Hi,

According to documents provided by editors of CLReq, the standard font size
of bopomofo in inter-character ruby is 30% of the base text, not 50% like
Japanese ruby.

The current WebKit implementation introduces a new keyword
-webkit-ruby-text, and interpret this to 25% when ruby-position is
inter-character, and 50% otherwise.

I suggest that this behavior have its place in the spec, but I'm not sure
what's the best way to do so.

- Xidorn