Allows embedders using the EmbedderHeapTracer to synchronously finalize
an already running garbage collection

Bug: chromium:843903
Tbr: ulan@chromium.org
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Iea01451ea2c1204c34dc7904732abae6b63e1704
Reviewed-on: https://chromium-review.googlesource.com/1128971
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54378}