Ignore "fake" window unload events in PortRPC

[robertknight]

In VitalSource custom/synthetic "unload" events are dispatched at the book's container frame when switching chapters. Make sure these don't trigger the code in the PortRPC class that is meant to be called when the window is unloaded.

These synthetic events were triggered after every chapter navigation. In Safari <= 15 the second and subsequent events would cause an error in the currentWindow.parent.postMessage call because the port had already been transferred after the first event was handled.

For context, see https://github.com/hypothesis/support/issues/161#issuecomment-2454560641.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.42%. Comparing base (2522dc1) to head (8b2fd90). Report is 9 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6652 +/- ## ======================================= Coverage 99.42% 99.42% ======================================= Files 270 270 Lines 10172 10174 +2 Branches 2418 2419 +1 ======================================= + Hits 10114 10116 +2 Misses 58 58 ```

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