Replace HeapType with a non-templated FieldType class.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 9771dcf729a788e108aa851e04146119acca93dc..f6f3287bfbe388c3782756d1ca99b1057e42310f 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2973,15 +2973,14 @@ int DescriptorArray::GetFieldIndex(int descriptor_number) {
   return GetDetails(descriptor_number).field_index();
 }
 
-
-HeapType* DescriptorArray::GetFieldType(int descriptor_number) {
+FieldType* DescriptorArray::GetFieldType(int descriptor_number) {
   DCHECK(GetDetails(descriptor_number).location() == kField);
   Object* value = GetValue(descriptor_number);
   if (value->IsWeakCell()) {
-    if (WeakCell::cast(value)->cleared()) return HeapType::None();
+    if (WeakCell::cast(value)->cleared()) return FieldType::None();
     value = WeakCell::cast(value)->value();
   }
-  return HeapType::cast(value);
+  return FieldType::cast(value);
 }