Fix inobject slack tracking for both subclassing and non-subclassing cases.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 24c8c14f1ced7a1b8a266342cf287cf81f22b18a..2eb0073ef6558f53dd9d6fe23dbd4b3b0be8a3a6 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -6161,9 +6161,25 @@ bool JSFunction::IsInOptimizationQueue() {
 }
 
 
-bool JSFunction::IsInobjectSlackTrackingInProgress() {
-  return has_initial_map() &&
-         initial_map()->counter() >= Map::kSlackTrackingCounterEnd;
+void JSFunction::CompleteInobjectSlackTrackingIfActive() {
+  if (has_initial_map() && initial_map()->IsInobjectSlackTrackingInProgress()) {
+    initial_map()->CompleteInobjectSlackTracking();
+  }
+}
+
+
+bool Map::IsInobjectSlackTrackingInProgress() {
+  return counter() >= Map::kSlackTrackingCounterEnd;
+}
+
+
+void Map::InobjectSlackTrackingStep() {
+  if (!IsInobjectSlackTrackingInProgress()) return;
+  int counter = this->counter();
+  set_counter(counter - 1);
+  if (counter == kSlackTrackingCounterEnd) {
+    CompleteInobjectSlackTracking();
+  }
 }