This CL improves --track-retaining-path with incremental marking:

* Retainers need to be cleared *after* the GC and for full GCs only.
  Otherwise a scavenger that runs during incremental marking would
  clear all retainers.
* Register marked root objects in IncrementalMarking::MarkRoots with
  retaining_root_.

Change-Id: I18ab9da48fb7ac45f9ec0826334fd338dc202edc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3297893Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78056}