Adds getters for GC phases to be used by advanced embedders to ensure
and check consistency conditions as needed.

Bug: chromium:1056170
Change-Id: Ia0b219f838bf31f0edbfe40585b95bb5eafa734d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2658328Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72430}