[utilities] Use Set instead of Array in ServiceWorkerCacheTreeElement
[utilities] Use Set instead of Array in ServiceWorkerCacheTreeElement

This CL is part of a cleanup effort to get rid of Array#remove.

R=17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org

Bug: chromium:1050549
Change-Id: I681f44bc21b33a8dda826fafd669028e6d9c11aa
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2084573
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

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