[system-health] Unify SH stories interface

tools/perf/page_sets/system_health/loading_stories.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import sys
-
 from page_sets.system_health import platforms
 from page_sets.system_health import system_health_story
 
 from page_sets.login_helpers import dropbox_login
 from page_sets.login_helpers import google_login
 
 
 class _LoadingStory(system_health_story.SystemHealthStory):
   """Abstract base class for single-page System Health user stories."""
-  pass
-
-
-def IterAllStoryClasses():
-  return system_health_story.IterAllStoryClasses(
-      sys.modules[__name__], _LoadingStory)
+  ABSTRACT_STORY = True
 
 
 ################################################################################
 # Search and e-commerce.
 ################################################################################
 
 
 class LoadGoogleStory(_LoadingStory):
   NAME = 'load:search:google'
   URL = 'https://www.google.com/#hl=en&q=science'
@@ skipping 332 matching lines @@
   URL = 'http://www.miniclip.com/games/en/'
   # Desktop only (requires Flash).
   SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
 
 
 class LoadAlphabettyStory(_LoadingStory):
   NAME = 'load:games:alphabetty'
   URL = 'https://king.com/play/alphabetty'
   # Desktop only (requires Flash).
   SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY