Before each `OneShotTimers` would have a `TimerListener` for its
lifetime, holding the timer `TaskSource`. The issue with this is that
the `TaskSource` for dedicated workers keeps the main thread object
alive, so as long as the `OneShotTimers` alive (until the worker thread
exists), the main thread object would never be garbage collected.
This change makes the creation of the listener on-demand, avoiding the
long-lived handle to the main thread object and slightly simplifying
`OneShotTimers` at the expense of some more operations when scheduling a
timer.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
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