[giow] (0) Allow TimeRanges to contain 'empty' ranges (not really empty, they contain one moment in time). Add some text admonishing implementations that don't allow one to seek arbitrarily when it's actually possible just inconvenient or slow.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26847
Affected topics: Video and Audio

git-svn-id: https://svn.whatwg.org/webapps@8819 340c8d12-0b0e-0410-8428-c7bf67bfef74