[Telemetry] Reenable long running backgrounded chrome telemetry test.

tools/perf/page_sets/system_health/long_running_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.login_helpers import google_login
 from page_sets.system_health import platforms
 from page_sets.system_health import system_health_story
 
-from telemetry import decorators
-
 
 IDLE_TIME_IN_SECONDS = 100
 SAMPLING_INTERVAL_IN_SECONDS = 1
 STEPS = IDLE_TIME_IN_SECONDS / SAMPLING_INTERVAL_IN_SECONDS
 
 
 class _LongRunningStory(system_health_story.SystemHealthStory):
   """Abstract base class for long running stories."""
   ABSTRACT_STORY = True
   BACKGROUND = False
@@ skipping 58 matching lines @@
 
 
 class LongRunningGmailMobileForegroundStory(_LongRunningGmailMobileBase):
   NAME = 'long_running:tools:gmail-foreground'
 
 
 class LongRunningGmailDesktopForegroundStory(_LongRunningGmailDesktopBase):
   NAME = 'long_running:tools:gmail-foreground'
 
 
-# crbug.com/643110
-@decorators.Disabled('android')
 class LongRunningGmailMobileBackgroundStory(_LongRunningGmailMobileBase):
   BACKGROUND = True
   NAME = 'long_running:tools:gmail-background'
 
 
 class LongRunningGmailDesktopBackgroundStory(_LongRunningGmailDesktopBase):
   BACKGROUND = True
   NAME = 'long_running:tools:gmail-background'