Add Javascript-Heavy tags for system health stories with heavy uses of JS

tools/perf/page_sets/system_health/browsing_stories.py

Index: tools/perf/page_sets/system_health/browsing_stories.py
diff --git a/tools/perf/page_sets/system_health/browsing_stories.py b/tools/perf/page_sets/system_health/browsing_stories.py
index d509d5dcc7b8e2fb1d91b023090a79c73e202289..d22ac3519516510595c56a330f0a17f6c93bd1e5 100644
--- a/tools/perf/page_sets/system_health/browsing_stories.py
+++ b/tools/perf/page_sets/system_health/browsing_stories.py
@@ -4,6 +4,7 @@
 # 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
@@ -99,6 +100,7 @@ class CnnStory(_NewsBrowsingStory):
   URL = 'http://edition.cnn.com/'
   ITEM_SELECTOR = '.cd__content > h3 > a'
   ITEMS_TO_VISIT = 2
+  TAGS = [story_tags.JAVASCRIPT_HEAVY]
 
 
 class FacebookMobileStory(_NewsBrowsingStory):
@@ -394,6 +396,7 @@ class YouTubeMobileStory(_MediaBrowsingStory):
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY
   IS_SINGLE_PAGE_APP = True
   ITEM_SELECTOR_INDEX = 3
+  TAGS = [story_tags.JAVASCRIPT_HEAVY]
 
 
 class YouTubeDesktopStory(_MediaBrowsingStory):
@@ -407,6 +410,7 @@ class YouTubeDesktopStory(_MediaBrowsingStory):
   ITEMS_TO_VISIT = 8
   ITEM_SELECTOR_INDEX = 3
   PLATFORM_SPECIFIC = True
+  TAGS = [story_tags.JAVASCRIPT_HEAVY]
 
 
 class FacebookPhotosMobileStory(_MediaBrowsingStory):