[e] (0) Simplify how we define how to get chapters out of WebVTT
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=27102
Affected topics: Video Text Tracks

git-svn-id: https://svn.whatwg.org/webapps@8859 340c8d12-0b0e-0410-8428-c7bf67bfef74