This call can be used by embedder to request a GC for testing reasons.
The GC also takes the current embedder stack state as an argument that
is forwarded to the embedder when entering the atomic pause.

This way embedders can request garbage collections for testing and set
how the embedder should treat the stack.

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Id10604565b4457dd0fca402afeb5f8e592fa0bae
Reviewed-on: https://chromium-review.googlesource.com/1183431
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55285}