[Sync] Rely on directory to decide which types need to be purged

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index 2830eec445891b3e97f00c177583bb03b14370e6..01baac76b42369971ebdfbdb7792005ad720f9bf 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -109,6 +109,9 @@ class SYNC_EXPORT Directory {
     // a full download of all objects of the model will be initiated.
     void ResetDownloadProgress(ModelType model_type);
 
+    // Whether a valid progress marker exists for |model_type|.
+    bool HasEmptyDownloadProgress(ModelType model_type);
+
     // Last sync timestamp fetched from the server.
     sync_pb::DataTypeProgressMarker download_progress[MODEL_TYPE_COUNT];
     // Sync-side transaction version per data type. Monotonically incremented