This reverts commit 44a1ed8f.

Reason for revert: https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20gcc/6124?

Original change's description:
> cppgc: Integrate conservative stack scan into GC
> 
> With this change we support allocation of objects and keeping them alive via
> conservative stack scan.
> 
> Change-Id: Id94d7ced503ad0b1378643e0c13c2a8b65ad2327
> Bug: chromium:1056170
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135729
> Reviewed-by: Omer Katz <omerkatz@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Anton Bikineev <bikineev@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67008}

TBR=ulan@chromium.org,hpayer@chromium.org,mlippautz@chromium.org,bikineev@chromium.org,omerkatz@chromium.org

Change-Id: I71bf7a9b7f54f0ec8d64b1b1781699adb0feb138
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2137401Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67009}