Fix --trace-gc-object-stats

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}

src/heap/object-stats.cc

@@ -102,6 +102,10 @@ class FieldStatsCollector : public ObjectVisitor {
     *tagged_fields_count_ += 1;
   }
 
+  void VisitMapPointer(HeapObject host) override {
+    // Just do nothing, but avoid the inherited UNREACHABLE implementation.
+  }
+
  private:
   struct JSObjectFieldStats {
     JSObjectFieldStats() : embedded_fields_count_(0), smi_fields_count_(0) {}