Turns out we have a duplicate VoidFunction callback in the WebIDLs. Per
Mozilla bug 1324169, we should remove this callback from
RTCPeerConnection.webidl and let Function.webidl handle it instead.
The AttributeError faced by the new Basilisk dev wasn't helpful at all;
and turns out Mozilla had this exact same issue on bug 1505504. We're
also backporting that just in case we encounter another case of
callback duplication.
Also doing a minor fix in the MicroTaskRunnable class which Moonchild
forgot to do while working on Issue #1895.
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