It currently resides in MediaResource.h, but it's only used in a
ChannelMediaDecoder and ChannelMediaResource, so it doesn't need to be in the
abstract header.
MozReview-Commit-ID: GskE5mjMav1
--HG--
extra : rebase_source : b42321ddcb1966d4de7203f04852c14f6d271acd