Adjustments based on code review feedback

workers/manifestbackfillworker.py

@@ -83,13 +83,17 @@ class BrokenManifest(ManifestInterface):
     return self
     
   @property
-  def schema_version():
+  def schema_version(self):
     return 1
 
   @property
-  def layers_compressed_size():
+  def layers_compressed_size(self):
     return None
 
+  @property
+  def is_manifest_list(self):
+    return False
+
 
 class ManifestBackfillWorker(Worker):
   def __init__(self):