This helper class allows us to create simple channels that open an underlying
input stream when the channel is first opened, using a closure function,
without having to expose the nsBaseChannel class to non-necko code.
MozReview-Commit-ID: 6jiImdMXUp0
--HG--
extra : rebase_source : b8d30a28593940c41109b1352eb0c549213c5f89
Kris Maglione