We started using AbortController in HlsParser in 777c27ee, but that
change was made while the Tizen TV in our lab was offline. This restores
functionality to Tizen.
This polyfill uses getters, a feature of ES6 that the compiler won't
transpile into ES3, so this also changes the output language to ES5.
This should not be an issue, since ES5 has been well supported in all
browsers since IE10.