Only nsOggReader uses nsBuiltinDecoderReader::mDataOffset, so we should push it down into nsOggReader.
I've spun this off from bug 566779. I have a reviewed patch in that bug that I want to land now so I can base other work on top of it.
Created attachment 526152 [details] [diff] [review]
Carrying forward r=doublec from review in bug 566779.