WebVTT: Support fractional percentage values for position/line/size

This updates the WebVTT (cue settings) parser to accept fractional
percentage values for the 'position', 'line' and 'size' settings -
per the current spec draft.
Also updates spec steps and rewraps some comments.

BUG=364697

Review URL: https://codereview.chromium.org/864043002

git-svn-id: svn://svn.chromium.org/blink/trunk@188814 bbb929c8-8fbe-4397-9dbb-9b2b20218538
7 files changed
tree: 6240e4da69e046bf73a24b8d724d936d437ae39f
  1. third_party/