nsTimerEvent goes through a multi-step initialization for reasons that
are lost to time. We are also seeing peculiar crashes in
`nsTimerEvent::SetTimer()` that are only explainable by `SetTimer`
finding a non-null pointer where there should have been a null pointer.
The compiler ought to have been able to optimize those bits away, but no
matter: we can do the job ourselves and make the code clearer.
Since we only call `SetTimer` once, we should just move its work into
nsTimerEvent's constructor.
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