[Sync] Start implementation of migration for USS.

components/sync/engine_impl/model_type_worker.cc

Index: components/sync/engine_impl/model_type_worker.cc
diff --git a/components/sync/engine_impl/model_type_worker.cc b/components/sync/engine_impl/model_type_worker.cc
index 25422f64b1d5263f3a9f4e73fd65495db997067b..f3d81321206af51b331713e5c93664fac2ca58c1 100644
--- a/components/sync/engine_impl/model_type_worker.cc
+++ b/components/sync/engine_impl/model_type_worker.cc
@@ -27,6 +27,7 @@ namespace syncer {
 ModelTypeWorker::ModelTypeWorker(
     ModelType type,
     const sync_pb::ModelTypeState& initial_state,
+    bool trigger_initial_sync,
     std::unique_ptr<Cryptographer> cryptographer,
     NudgeHandler* nudge_handler,
     std::unique_ptr<ModelTypeProcessor> model_type_processor)
@@ -39,7 +40,7 @@ ModelTypeWorker::ModelTypeWorker(
   DCHECK(model_type_processor_);
 
   // Request an initial sync if it hasn't been completed yet.
-  if (!model_type_state_.initial_sync_done()) {
+  if (trigger_initial_sync) {
     nudge_handler_->NudgeForInitialDownload(type_);
   }