When enabled with the v8_enable_conservative_stack_scanning flag, a
snapshot of the call stack upon entry to GC is used to determine part of
the root-set. When the collector walks the stack, it looks at each value
and determines whether it could be a potential on-heap object pointer.

This is very experimental. For conservative stack scanning to work,
direct handles must be implemented.

Bug: v8:10614
Change-Id: Id4209cfbe76ef02239c903fabcb7f677b32fc977
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2375201
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69644}