[system-health] Add support for disabling individual stories on individual platforms

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.
 
 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
 
@@ skipping 145 matching lines @@
   URL = 'https://www.reddit.com/r/news/top/?sort=top&t=week'
   SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
 
 
 class LoadRedditMobileStory(_LoadingStory):
   NAME = 'load:news:reddit'
   URL = 'https://www.reddit.com/r/news/top/?sort=top&t=week'
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
 
 
-class LoadSohuStory(_LoadingStory):
+class LoadSohuMobileStory(_LoadingStory):
   NAME = 'load:news:sohu'
   # Using "https://" leads to missing images and scripts on mobile (due to
   # mixed content).
   URL = 'http://m.sohu.com/n/447433356/'
   # The desktop page (http://news.sohu.com/20160503/n447433356.shtml) almost
   # always fails to completely load due to
   # https://github.com/chromium/web-page-replay/issues/74.
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
 
 
@@ skipping 205 matching lines @@
     # The background of the game canvas is set when the "Tap screen to play"
     # caption is displayed.
     action_runner.WaitForJavaScriptCondition(
         'document.querySelector("#game canvas").style.background !== ""')
 
 
 class LoadMiniclipStory(_LoadingStory):
   NAME = 'load:games:miniclip'
   # Using "https://" causes "404 Not Found" during WPR recording.
   URL = 'http://www.miniclip.com/games/en/'
-  # Desktop only (requires Flash).
-  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
+  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY  # Requires Flash.
 
 
 class LoadAlphabettyStory(_LoadingStory):
   NAME = 'load:games:alphabetty'
   URL = 'https://king.com/play/alphabetty'
-  # Desktop only (requires Flash).
-  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
+  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY  # Requires Flash.