In many places we tried to guess the encoding of a piece of text.
This guess fails for Chinese UTF-8 text, and probably text in many
other languages.
However, DASH manifests, TTML files, WebVTT files, and VTTC box
payloads are all specified to be in UTF-8. Rather than guess and
possibly fail, treat all text in these contexts as UTF-8.
Change-Id: I00c652a9f1dd20855e94abfac84275e41dd9e266