Messageport: use a microtask to "maybe add pending port"

[gterzian]

The global, when first noting a transfer-received port, queues a task to then run the "maybe add pending port logic". The purpose of the task is to only add the port if it isn't transferred in the same task in which it is received.

servo/components/script/dom/globalscope.rs

Line 659 in f3b1f0d

// Queue a task to complete the transfer,

Queuing a task does leave open the possibility for JS to run and transfer the port again, before the queued task actually run. This can happen with timers, or UI events, which currently dispatch JS events without going through the task-queue and hence can be run before a task scheduled on the task-queue(see also #24737).

This could be made more robust by queuing a micro-task, or perhaps not queuing anything at all in the light of the changes introduced by #24664