Reland "In-object double fields unboxing (for 64-bit only)."

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index f89a19fd4a9b06abf2b1dd4e17a86ef09d38cdd0..1ec6873b1104027cb0396b28fa4129f07068cb05 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -652,8 +652,15 @@ BasicJsonStringifier::Result BasicJsonStringifier::SerializeJSObject(
       if (details.IsDontEnum()) continue;
       Handle<Object> property;
       if (details.type() == FIELD && *map == object->map()) {
-        property = Handle<Object>(object->RawFastPropertyAt(
-            FieldIndex::ForDescriptor(*map, i)), isolate_);
+        FieldIndex field_index = FieldIndex::ForDescriptor(*map, i);
+        Isolate* isolate = object->GetIsolate();
+        if (object->IsUnboxedDoubleField(field_index)) {
+          double value = object->RawFastDoublePropertyAt(field_index);
+          property = isolate->factory()->NewHeapNumber(value);
+
+        } else {
+          property = handle(object->RawFastPropertyAt(field_index), isolate);
+        }
       } else {
         ASSIGN_RETURN_ON_EXCEPTION_VALUE(
             isolate_, property,