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

This CL changes ResourceTreeModel._childFrames to be a Set instead
an Array. This is part of a cleanup effort to get rid of Array#remove.

Bug: chromium:1050549
Change-Id: I8718dc149ec9fe75fa38dddefe59625a5d63eb26
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2095311
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>

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