Preserve flags for GC finalization triggered by EmbedderHeapTracer.

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ie3d039bc1b69771ce9830dddb5e4d4375581981f
Reviewed-on: https://chromium-review.googlesource.com/1134766Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54391}