Revert "[heap, runtime] Fix data race in prototype map transition during"

This reverts commit b2d1f272.

Reason for revert: assertion failure

Original change's description:
> [heap, runtime] Fix data race in prototype map transition during
> concurrent marking.
> 
> BUG=chromium:694255
> 
> Change-Id: I172167623e9deab692fb506d7d4211d210b09a80
> Reviewed-on: https://chromium-review.googlesource.com/579092
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46813}

TBR=ulan@chromium.org,ishell@chromium.org

Change-Id: Ida5c66c3e880b9a03ffacbc6f32b1d5b2cfc8260
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/581287Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46815}

src/heap/heap.cc

@@ -4522,8 +4522,7 @@ void Heap::NotifyObjectLayoutChange(HeapObject* object,
     incremental_marking()->MarkBlackAndPush(object);
   }
 #ifdef VERIFY_HEAP
-  DCHECK(pending_layout_change_object_ == nullptr ||
-         pending_layout_change_object_ == object);
+  DCHECK(pending_layout_change_object_ == nullptr);
   pending_layout_change_object_ = object;
 #endif
 }

src/objects.cc

@@ -4129,13 +4129,6 @@ void JSObject::MigrateToMap(Handle<JSObject> object, Handle<Map> new_map,
     // Slow-to-slow migration is trivial.
     object->synchronized_set_map(*new_map);
   } else if (!new_map->is_dictionary_map()) {
-    if (old_map->is_prototype_map()) {
-      DisallowHeapAllocation no_allocation;
-      // Ensure that the object is marked because its old map is going
-      // to drop the descriptor array and the layout descriptor, which
-      // is unsafe for the concurrent marker.
-      object->GetHeap()->NotifyObjectLayoutChange(*object, no_allocation);
-    }
     MigrateFastToFast(object, new_map);
     if (old_map->is_prototype_map()) {
       DCHECK(!old_map->is_stable());