Unify percentage syntax and parsing.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=19836

Introducing double percentage values for "line", "size" and "position"
cue settings.

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.

Update VTTCue DOM API with double values for "line", "size", and
"position" percentage values.