Attempting to trace object stats ran into an UNREACHABLE because
the FieldStatsCollector didn't override VisitMapPointer.

Change-Id: I38f2739c204e462e5a35f2a7ca6e0f100161b240
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2905604
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74679}