[Sync:USS] DCHECK bookmarks have valid unique_position in directory

uss_migrator is responsible for migration (meta)data from the directory
to the USS architecture upon first time switching to USS.
USS assumes that all bookmarks have valid unique_position.
This CL adds a DCHECK that this is actually the case.
Directory is responsible for assigning valid unique positions for
legacy bookmarks that are missing the field.

Bug: 516866
Change-Id: Ib963016a56c4c9275649a1ff78f440d7fbe23023
Reviewed-on: https://chromium-review.googlesource.com/1240296
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593548}

components/sync/engine_impl/uss_migrator.cc

@@ -23,7 +23,8 @@ namespace syncer {
 
 namespace {
 
-bool ExtractSyncEntity(ReadTransaction* trans,
+bool ExtractSyncEntity(ModelType type,
+                       ReadTransaction* trans,
                        int64_t id,
                        sync_pb::SyncEntity* entity) {
   ReadNode read_node(trans);
@@ -50,6 +51,11 @@ bool ExtractSyncEntity(ReadTransaction* trans,
   if (entry.GetServerUniquePosition().IsValid()) {
     *entity->mutable_unique_position() =
         entry.GetServerUniquePosition().ToProto();
+  } else {
+    // All boookmarks should have valid unique_positions including legacy
+    // bookmarks that are missing the field. Directory should have taken care of
+    // assigning proper unique_position during the first sync flow.
+    DCHECK_NE(BOOKMARKS, type);
   }
   entity->set_server_defined_unique_tag(entry.GetUniqueServerTag());
 
@@ -103,7 +109,7 @@ bool MigrateDirectoryDataWithBatchSize(ModelType type,
     const size_t batch_limit = std::min(i + batch_size, child_ids.size());
     for (; i < batch_limit; i++) {
       auto entity = std::make_unique<sync_pb::SyncEntity>();
-      if (!ExtractSyncEntity(&trans, child_ids[i], entity.get())) {
+      if (!ExtractSyncEntity(type, &trans, child_ids[i], entity.get())) {
         LOG(ERROR) << "Failed to fetch child node for "
                    << ModelTypeToString(type);
         // Inform the worker so it can clear any partial data and trigger a