Introduce RootVisitor and related class hierarchy to just handle
roots. This avoids the awkard definitions for roots visiation in all
the cases they are not needed.

Change-Id: Ib0912e4bf543db2ecf68caead6929c68d6afdda6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3782794Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82107}