Namely, will_unblock_immediately_without_blocking(Reason).
This virtual function is called on a blocker *before any block occurs*,
if it turns out that we don't need to block the thread after all.
This can happens for one of two reasons:
- UnblockImmediatelyReason::UnblockConditionAlreadyMet
We don't need to block the thread because the condition for
unblocking it is already met.
- UnblockImmediatelyReason::TimeoutInThePast
We don't need to block the thread because a timeout was specified
and that timeout is already in the past.
This patch does not introduce any behavior changes, it's only meant to
clarify this part of the blocking logic.
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