Adds a heap verification GN arg to gate the marking verifier and live
bytes verification on. The flag may be used in future for other more
expensive checks as well.

Currently, the flag is automatically enabled in dcheck_is_on and debug
builds.

The change enables live bytes verification for the library in regular
debug builds which may flush out issues.

Bug: v8:11785
Change-Id: I0f41bc0d76ebea9f6a8c9315c947598015ee5d68
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097868
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76325}