So it is callable from non-main thread.
MozReview-Commit-ID: atYmz4u2c9
--HG--
extra : rebase_source : 2e10064730b3e7e1ecb1a4fd65cf2e2da0390290
extra : source : 5680a6942f6985f9c6bbf284a9768ab910b37804
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