Revert of [tools/perf] Disable browse:social:twitter on mac

tools/perf/page_sets/system_health/browsing_stories.py

 # encoding: utf-8
 # 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 story_tags
 from page_sets.system_health import system_health_story
 
 from page_sets.login_helpers import pinterest_login
@@ skipping 181 matching lines @@
 
 class RedditMobileStory(_ArticleBrowsingStory):
   """The top website in http://www.alexa.com/topsites/category/News"""
   NAME = 'browse:news:reddit'
   URL = 'https://www.reddit.com/r/news/top/?sort=top&t=week'
   IS_SINGLE_PAGE_APP = True
   ITEM_SELECTOR = '.PostHeader__post-title-line'
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
 
 
-@decorators.Disabled('mac')  # crbug.com/722094
 class TwitterMobileStory(_ArticleBrowsingStory):
   NAME = 'browse:social:twitter'
   URL = 'https://www.twitter.com/nasa'
   ITEM_SELECTOR = '.Tweet-text'
   CONTAINER_SELECTOR = '.NavigationSheet'
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
 
 
 @decorators.Disabled('win')  # crbug.com/662971
 class TwitterDesktopStory(_ArticleBrowsingStory):
@@ skipping 551 matching lines @@
     # Reduce the speed (the current wpr is recorded with speed set to 50)  when
     # recording the wpr. If we scroll too fast, the data will not be recorded
     # well. After recording reset it back to the original value to have a more
     # realistic scroll.
     action_runner.RepeatableBrowserDrivenScroll(
         x_scroll_distance_ratio = 0.0, y_scroll_distance_ratio = 1,
         repeat_count=3, speed=400, timeout=120)
     action_runner.RepeatableBrowserDrivenScroll(
         x_scroll_distance_ratio = 1, y_scroll_distance_ratio = 0,
         repeat_count=3, speed=500, timeout=120)