Drive-by-fix: enable heap verification in mksnapshot.

Bug: chromium:1025468
Change-Id: Ieb52d5139fa37df4ff0d8e8d46c3e0e6d14c2c8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924363Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65102}