[perf] Add browse:social:twitter:2018 system health story

Bug: 878390
Change-Id: I9415f702715ec0332720829f2e385f3787d77ec7
Reviewed-on: https://chromium-review.googlesource.com/c/1261982Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599971}

tools/perf/page_sets/data/system_health_desktop.json

@@ -72,6 +72,9 @@
         "browse:social:twitter": {
             "DEFAULT": "system_health_desktop_053.wprgo"
         },
+        "browse:social:twitter:2018": {
+            "DEFAULT": "system_health_desktop_6bd6f0c70a.wprgo"
+        },
         "browse:social:twitter_infinite_scroll": {
             "DEFAULT": "system_health_desktop_054.wprgo"
         },

tools/perf/page_sets/data/system_health_desktop_6bd6f0c70a.wprgo.sha1

+6bd6f0c70a9cfaaea17461a177095e9374754e69
\ No newline at end of file

tools/perf/page_sets/system_health/browsing_stories.py

@@ -286,6 +286,15 @@ class TwitterDesktopStory(_ArticleBrowsingStory):
   TAGS = [story_tags.YEAR_2016]
 
 
+class TwitterDesktopStory2018(_ArticleBrowsingStory):
+  NAME = 'browse:social:twitter:2018'
+  URL = 'https://www.twitter.com/nasa'
+  IS_SINGLE_PAGE_APP = True
+  ITEM_SELECTOR = '.tweet-text'
+  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
+  TAGS = [story_tags.YEAR_2018]
+
+
 class WashingtonPostMobileStory(_ArticleBrowsingStory):
   """Progressive website"""
   NAME = 'browse:news:washingtonpost'