Get rid of ConvertFieldToDescriptor and simplify related code.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index ad5b0e3e5a87dcf0a9e1e28e2d348cc7d44ecd34..c5631b14e7750aa089eba7d59dc1234d9ea2f002 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1865,14 +1865,15 @@ bool JSObject::HasFastProperties() {
 }
 
 
-bool JSObject::TooManyFastProperties(int properties,
-                                     JSObject::StoreFromKeyed store_mode) {
+bool JSObject::TooManyFastProperties(StoreFromKeyed store_mode) {
   // Allow extra fast properties if the object has more than
-  // kFastPropertiesSoftLimit in-object properties. When this is the case,
-  // it is very unlikely that the object is being used as a dictionary
-  // and there is a good chance that allowing more map transitions
-  // will be worth it.
-  int inobject = map()->inobject_properties();
+  // kFastPropertiesSoftLimit in-object properties. When this is the case, it is
+  // very unlikely that the object is being used as a dictionary and there is a
+  // good chance that allowing more map transitions will be worth it.
+  Map* map = this->map();
+  if (map->unused_property_fields() != 0) return false;
+
+  int inobject = map->inobject_properties();
 
   int limit;
   if (store_mode == CERTAINLY_NOT_STORE_FROM_KEYED) {
@@ -1880,7 +1881,7 @@ bool JSObject::TooManyFastProperties(int properties,
   } else {
     limit = Max(inobject, kFastPropertiesSoftLimit);
   }
-  return properties > limit;
+  return properties()->length() > limit;
 }