mre: chrome does not gc filereaders quickly enough inside webworkers

select a file around 16 MiB large: 

screenshot after selecting Windows.UI.Xaml.dll (17'110 KiB)
which should have consumed 16 * 16 = ~256 MiB RAM at most

this bug is triggered by repeatedly creating filereaders in workers;
reusing a single filereader makes the problem MOSTLY go away --
see `worker-gc.js` to toggle this behavior

observed on:
* win10 21h2 (19044.1889) + chr 103.0.5060.66 (old stable gchrome)
* win10 1809 (17763.3046) + chr 106.0.5247.0 (build 1036146, dc@appspot)
* win10 1809 (17763.316) + chr 106.0.5247.0 (build 1036146, dc@appspot)
* win10 1809 (17763.316) + chr 105.0.5195.37 (latest official gchrome beta)

NOT observed on:
* debian 11.4 + chr 104.0.5112.79
* win10 + firefox

(17763.316 = stock LTSC-2019 without any updates)