Refactor serving_dirs.

tools/telemetry/telemetry/user_story/user_story_set.py

Index: tools/telemetry/telemetry/user_story/user_story_set.py
diff --git a/tools/telemetry/telemetry/user_story/user_story_set.py b/tools/telemetry/telemetry/user_story/user_story_set.py
index b4315ffc9e7799afe216fa37bcf115909d0daace..a239a0c425ceb7b50a50e8acebef07bb18582ecd 100644
--- a/tools/telemetry/telemetry/user_story/user_story_set.py
+++ b/tools/telemetry/telemetry/user_story/user_story_set.py
@@ -27,6 +27,9 @@ class UserStorySet(object):
           Web Page Replay's archive data. Valid values are: None,
           PUBLIC_BUCKET, PARTNER_BUCKET, or INTERNAL_BUCKET (defined
           in telemetry.util.cloud_storage).
+      serving_dirs: A set of paths, relative to self.base_dir, to directories
+          containing hash files for non-wpr archive data stored in cloud
+          storage.
     """
     self.user_stories = []
     self._archive_data_file = archive_data_file
@@ -48,7 +51,11 @@ class UserStorySet(object):
 
   @property
   def serving_dirs(self):
-    return self._serving_dirs
+    all_serving_dirs = self._serving_dirs.copy()
+    for user_story in self.user_stories:
+      if user_story.serving_dir:
+        all_serving_dirs.add(user_story.serving_dir)
+    return all_serving_dirs
 
   @property
   def archive_data_file(self):