Omit user roots when raw heap snapshots are used, i.e., when
the gn flag v8_enable_raw_heap_snapshots is enabled. For regular
Chrome production builds this is not the case.

Blink CL: https://crrev.com/c/1529096

Bug: chromium:936797
Change-Id: I5ae0ec1ecfab9a76352d8ce927d1c40e707262cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1528994
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60351}