[Sync] ModelTypeWorker should delete tracker for entity deleted on the server

components/sync/engine_impl/worker_entity_tracker.cc

Index: components/sync/engine_impl/worker_entity_tracker.cc
diff --git a/components/sync/engine_impl/worker_entity_tracker.cc b/components/sync/engine_impl/worker_entity_tracker.cc
index 8632e291579bc76960c53f2c65d1817a947b45f0..24b306a6a555ed682e5dd1857cba7a4b448d65b7 100644
--- a/components/sync/engine_impl/worker_entity_tracker.cc
+++ b/components/sync/engine_impl/worker_entity_tracker.cc
@@ -25,6 +25,11 @@ bool WorkerEntityTracker::HasPendingCommit() const {
   return !!pending_commit_;
 }
 
+bool WorkerEntityTracker::IsTombstone() const {
+  DCHECK(pending_commit_);
+  return pending_commit_->entity.value().is_deleted();
+}
+
 void WorkerEntityTracker::PopulateCommitProto(
     sync_pb::SyncEntity* commit_entity) const {
   DCHECK(HasPendingCommit());