The API will be used to post GC tasks that benefit from being executed
at top level.

Bug: chromium:843903
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I062855e810ca9a8d8af8ae8b66e02c85e108798b
Reviewed-on: https://chromium-review.googlesource.com/c/1273045Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56528}