[autotest] Allow fields of SERIALIZATION_LINKS_TO_KEEP to be serialized

In the logic of ModelExtensions, fields in SERIALIZATION_LINKS_TO_KEEP
are not considered to be related fields.
These fields can be included in serialization without any break of
logic.
This fix allows syncing back host_id of afe_host_queue_entries
from shard to master via heartbeat.

BUG=chromium:461637
DEPLOY=apache,scheduler,shard-client
TEST=puppylab. Run a dummy suite on a lumpy host (in shard)
and check master DB for the suite jobs' HQE after completion.

Change-Id: I6369f88106d9489385f0d7b78f10a3b0bf5f4314
Reviewed-on: https://chromium-review.googlesource.com/257031
Tested-by: Mungyung Ryu <mkryu@google.com>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Mungyung Ryu <mkryu@google.com>
diff --git a/frontend/afe/model_logic.py b/frontend/afe/model_logic.py
index 5d98e09..84033f1 100644
--- a/frontend/afe/model_logic.py
+++ b/frontend/afe/model_logic.py
@@ -948,8 +948,7 @@
             for field in self._meta.concrete_model._meta.local_fields:
                 if field.rel is None:
                     serialized[field.name] = field._get_val_from_obj(self)
-                elif (include_dependencies and
-                      field.name in self.SERIALIZATION_LINKS_TO_KEEP):
+                elif field.name in self.SERIALIZATION_LINKS_TO_KEEP:
                     # attname will contain "_id" suffix for foreign keys,
                     # e.g. HostAttribute.host will be serialized as 'host_id'.
                     # Use it for easy deserialization.