[interpreter] Properly collect for-in slow mode feedback.

src/type-feedback-vector-inl.h

Index: src/type-feedback-vector-inl.h
diff --git a/src/type-feedback-vector-inl.h b/src/type-feedback-vector-inl.h
index 97df1b9ae994abac87a7615d865a08bede05192c..015104e96a538a396a515f38c98faa7d4b4cc306 100644
--- a/src/type-feedback-vector-inl.h
+++ b/src/type-feedback-vector-inl.h
@@ -133,23 +133,19 @@ void TypeFeedbackVector::ComputeCounts(int* with_type_info, int* generic) {
   *generic = gen;
 }
 
-
-Handle<Object> TypeFeedbackVector::UninitializedSentinel(Isolate* isolate) {
+Handle<Symbol> TypeFeedbackVector::UninitializedSentinel(Isolate* isolate) {
   return isolate->factory()->uninitialized_symbol();
 }
 
-
-Handle<Object> TypeFeedbackVector::MegamorphicSentinel(Isolate* isolate) {
+Handle<Symbol> TypeFeedbackVector::MegamorphicSentinel(Isolate* isolate) {
   return isolate->factory()->megamorphic_symbol();
 }
 
-
-Handle<Object> TypeFeedbackVector::PremonomorphicSentinel(Isolate* isolate) {
+Handle<Symbol> TypeFeedbackVector::PremonomorphicSentinel(Isolate* isolate) {
   return isolate->factory()->premonomorphic_symbol();
 }
 
-
-Object* TypeFeedbackVector::RawUninitializedSentinel(Isolate* isolate) {
+Symbol* TypeFeedbackVector::RawUninitializedSentinel(Isolate* isolate) {
   return isolate->heap()->uninitialized_symbol();
 }