Remove deepActiveElement from prototype
Remove deepActiveElement from prototype

Now the upstream layout tests have been updated to use the new platform
utility, we can remove the prototype extension.

Bug: 1050549
Change-Id: I1eecbd478b4dc5daf156cf732307a921c7d526e3
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3627331
Reviewed-by: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Commit-Queue: Simon Zünd <38944668e9e2ca98ccd0275b53370f90fa55c113@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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