Allows embedders to allocate C++ objects on the internal managed C++
heap.

Bug: chromium:1056170
Change-Id: Ibd81d0fc915478a81f14e8ab12a631e442790f04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2536642Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71225}