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 1c345f58de07aa98c19891fcd0d41b0d852d7cc8..4dd39206177373a834610f6f5c053bec50a13994 100644
--- a/tools/perf/page_sets/system_health/story_tags.py
+++ b/tools/perf/page_sets/system_health/story_tags.py
@@ -25,6 +25,10 @@ IMAGES = Tag(
 INTERNATIONAL = Tag(
     'international', 'Story has navigations to websites with content in non '
     'English languages.')
+EMERGING_MARKET = Tag(
+    'emerging-market', 'Story that is important because of the usage in '
+    'emerging markets with low-end mobile devices and slow network '
+    'connections.')

[perezju, 2017/04/04 09:23:38]

Rephrase suggestion: Story has significant usage in emerging markets with
low-end mobile devices and slow network connections.

[ssid, 2017/04/04 20:59:50]

Done.

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