Add System health stories for Emerging market

tools/perf/page_sets/system_health/story_tags.py

Index: tools/perf/page_sets/system_health/story_tags.py
diff --git a/tools/perf/page_sets/system_health/story_tags.py b/tools/perf/page_sets/system_health/story_tags.py
index 71d5c51a6167542649f92cb4f921311511c40bb7..4458165bf7bdfed78cf57ee35f8e63a928b5b16d 100644
--- a/tools/perf/page_sets/system_health/story_tags.py
+++ b/tools/perf/page_sets/system_health/story_tags.py
@@ -25,6 +25,9 @@ IMAGES = Tag(
 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.')

[nednguyen, 2017/04/17 19:20:28]

hmhh, actually sorry that I didn't notice slow network connection part. You can
set the traffic shaping setting for a story with s.t like traffic_setting =
'3g'. But since this CL is already complicated & doing that would increases the
cycle time, it could be something to consider in a separate CL.

[ssid, 2017/04/17 20:37:05]

Files https://bugs.chromium.org/p/chromium/issues/detail?id=712288 to keep
track. Thanks!

 JAVASCRIPT_HEAVY = Tag(
     'javascript_heavy', 'Story has navigations to websites with heavy usages '
     'of JavaScript. The story uses 20Mb+ memory for javascript and local '