This allows MOZ_TRY and MOZ_TRY_VAR to be transparently used in XPCOM methods
when compatible Result types are used.
Also removes a compatibility macro in SimpleChannel.cpp, and an identical
specialization in AddonManagerStartup, which are no longer necessary after
this change.
MozReview-Commit-ID: 94iNrPDJEnN
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