Adds a public method that embedders can use to trigger garbage
collections. Such garbage collections are always required to have a
source and reason specifying which components calls it why.

Change-Id: I6ae983f99227febc1b7f0dd15c191d5b1eaaf3f3
Bug: chromium:1056170
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2181332Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67631}