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

Initial version shows predictably 29 network errors:
- 28x times https://abs.twimg.com/k/en-gb/13.hls.js.en-gb.43b6de858df39857f6b2.js
-  1x https://twitter.com/manifest.json

Bug: 878390
Change-Id: Ifa4dd5dae68f1450d47d22241f094a6e454593cd
Reviewed-on: https://chromium-review.googlesource.com/1219003
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595794}

tools/perf/page_sets/data/system_health_desktop.json

@@ -63,6 +63,9 @@
         "browse:social:twitter_infinite_scroll": {
             "DEFAULT": "system_health_desktop_054.wprgo"
         },
+        "browse:social:twitter_infinite_scroll:2018": {
+            "DEFAULT": "system_health_desktop_f6af33019a.wprgo"
+        },
         "browse:tech:discourse_infinite_scroll": {
             "DEFAULT": "system_health_desktop_058.wprgo"
         },

tools/perf/page_sets/data/system_health_desktop_f6af33019a.wprgo.sha1

+f6af33019ac05e1226cd1ed8ddfcb4ed31e083bd
\ No newline at end of file

tools/perf/page_sets/system_health/browsing_stories.py

@@ -1077,3 +1077,10 @@ class TwitterScrollDesktopStory(_InfiniteScrollStory):
   URL = 'https://twitter.com/taylorswift13'
   SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
   TAGS = [story_tags.INFINITE_SCROLL, story_tags.YEAR_2016]
+
+
+class TwitterScrollDesktopStory2018(_InfiniteScrollStory):
+  NAME = 'browse:social:twitter_infinite_scroll:2018'
+  URL = 'https://twitter.com/NASA'
+  SUPPORTED_PLATFORMS = platforms.DESKTOP_ONLY
+  TAGS = [story_tags.INFINITE_SCROLL, story_tags.YEAR_2018]