Unify percentage syntax and parsing.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=19836

In the syntax section: use "WebVTT percentage value" syntax for the percentages in "line", "size" and "position" cue settings.
In the parsing section: us "parse a percentage string" algo for the percentages in "line", "size" and "position" cue settings.