[system-health] Fix mobile facebook story for Nexus7v2.

Facebook fetches items on demand while scrolling. Before this patch
Nexus7v2 was timing out while waiting for items because the scroll
distance was not enough to trigger fetching of items.

This patch increases the scroll distance and updates WPR.

*** NOTE TO SHERRIF ***
Regressions are expected in browse:social:facebook story.

BUG=631022

Review-Url: https://codereview.chromium.org/2175383003
Cr-Commit-Position: refs/heads/master@{#407756}

tools/perf/page_sets/data/system_health_mobile.json

@@ -89,11 +89,11 @@
         "system_health_mobile_025.wpr": [
             "browse:news:flipboard"
         ],
-        "system_health_mobile_026.wpr": [
-            "browse:social:facebook"
-        ],
         "system_health_mobile_027.wpr": [
             "browse:news:reddit"
+        ],
+        "system_health_mobile_028.wpr": [
+            "browse:social:facebook"
         ]
     },
     "description": "Describes the Web Page Replay archives for a story set. Don't edit by hand! Use record_wpr for updating."

tools/perf/page_sets/data/system_health_mobile_028.wpr.sha1

+177860604865cf48ae6b60bff092679e7c32dfc2
\ No newline at end of file

tools/perf/page_sets/system_health/browsing_stories.py

@@ -94,6 +94,9 @@ class FacebookMobileStory(_NewsBrowsingStory):
   NAME = 'browse:social:facebook'
   URL = 'https://www.facebook.com/rihanna'
   ITEM_SELECTOR = 'article ._5msj'
+  # We scroll further than usual so that Facebook fetches enough items
+  # (crbug.com/631022)
+  MAIN_PAGE_SCROLL_REPEAT = 1
   SUPPORTED_PLATFORMS = platforms.MOBILE_ONLY