We send back input events handling results in batch. This reduces IPC,
and make the following **TODO** possible in a follow-up.
Testing: I hope existing tests results won't change.
Part of
https://github.com/servo/servo/issues/43136#issuecomment-4031915873
**TODO**: We have not done
> collect ids and send back a reply for each of them once the coalesced
event has been handled.
yet.
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Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>