The atomic marking phase was organized in many distinct smaller
phases. In particular, before http://crrev.com/c/3584115 the marking
phase split into two large separate phases.

This CL reorganizes marking into two phases that perform regular V8
heap marking, Oilpan, and ephemerons:
- A parallel phase that likely drains all marking worklists;
- A single-threaded final phase to catch any left overs;

This avoids artificial splitting in phases and also avoids repeated
starting and joining of jobs.

Change-Id: I5cccfc5777837d9ece10d8f4925781bf2d07d9da
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3602507Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80265}