[weakref] Fix MaybeObject conversion in ObjectStats

Fixes the MaybeObject->Object conversion in ObjectStats to allow Smis,
rather than just HeapObjects.

Change-Id: I845613c47bb6ca696d444a025100b471fb385980
Reviewed-on: https://chromium-review.googlesource.com/1049925Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53070}

src/heap/object-stats.cc

@@ -408,7 +408,7 @@ void ObjectStatsCollectorImpl::RecordVirtualJSObjectDetails(JSObject* object) {
 
 static ObjectStats::VirtualInstanceType GetFeedbackSlotType(
     MaybeObject* maybe_obj, FeedbackSlotKind kind, Isolate* isolate) {
-  HeapObject* obj = maybe_obj->ToStrongHeapObject();
+  Object* obj = maybe_obj->GetHeapObjectOrSmi();
   switch (kind) {
     case FeedbackSlotKind::kCall:
       if (obj == *isolate->factory()->uninitialized_symbol() ||