[runtime] Do not update visitor id on completion of slack tracking.

Visitor id for JSObjects does not depend on the instance size.

BUG=chromium:694255

Change-Id: I6b4b559207609f4a4c19373572be6692a55c5187
Reviewed-on: https://chromium-review.googlesource.com/520763Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45650}

src/objects.cc

@@ -12128,14 +12128,15 @@ static void GetMinInobjectSlack(Map* map, void* data) {
 
 
 static void ShrinkInstanceSize(Map* map, void* data) {
+#ifdef DEBUG
+  int old_visitor_id = Heap::GetStaticVisitorIdForMap(map);
+#endif
   int slack = *reinterpret_cast<int*>(data);
   map->SetInObjectProperties(map->GetInObjectProperties() - slack);
   map->set_unused_property_fields(map->unused_property_fields() - slack);
   map->set_instance_size(map->instance_size() - slack * kPointerSize);
   map->set_construction_counter(Map::kNoSlackTracking);
-
-  // Visitor id might depend on the instance size, recalculate it.
-  map->set_visitor_id(Heap::GetStaticVisitorIdForMap(map));
+  DCHECK_EQ(old_visitor_id, Heap::GetStaticVisitorIdForMap(map));
 }
 
 static void StopSlackTracking(Map* map, void* data) {