Add System health stories for Emerging market

tools/perf/page_sets/system_health/story_tags.py

 # Copyright 2017 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 collections
 
 
 Tag = collections.namedtuple('Tag', ['name', 'description'])
 
 
 # Below are tags that describe various aspect of system health stories.
 # A story can have multiple tags. All the tags should be noun.
 
 AUDIO_PLAYBACK = Tag(
     'audio-playback', 'Story has audio playing.')
 CANVAS_ANIMATION = Tag(
     'canvas-animation', 'Story has animations that are implemented using '
     'html5 canvas.')
 CSS_ANIMATION = Tag(
     'css-animation', 'Story has animations that are implemented using CSS.')
 EXTENSION = Tag(
     'extension', 'Story has browser with extension installed.')
 IMAGES = Tag(
     'images', 'Story has sites with heavy uses of images.')
 INTERNATIONAL = Tag(
     'international', 'Story has navigations to websites with content in non '
     'English languages.')
+EMERGING_MARKET = Tag(
+    'emerging-market', 'Story has significant usage in emerging markets with '
+    'low-end mobile devices and slow network connections.')
 JAVASCRIPT_HEAVY = Tag(
     'javascript-heavy', 'Story has navigations to websites with heavy usages '
     'of JavaScript. The story uses 20Mb+ memory for javascript and local '
     'run with "v8" category enabled also shows the trace has js slices across '
     'the whole run.')
 SCROLL = Tag(
     'scroll', 'Story has scroll gestures & scroll animation.')
 PINCH_ZOOM = Tag(
     'pinch-zoom', 'Story has pinch zoom gestures & pinch zoom animation.')
 TABS_SWITCHING = Tag(
@@ skipping 13 matching lines @@
   # duplicate tag names.
   for obj in globals().values():
     if isinstance(obj, Tag):
       all_tags.append(obj)
       assert obj.name not in all_tag_names, 'Duplicate tag name: %s' % obj.name
       all_tag_names.add(obj.name)
   return all_tags
 
 
 ALL_TAGS = _ExtractAllTags()