All graphs for this bug:
  https://chromeperf.appspot.com/group_report?bug_id=846836

(For debugging:) Original alerts at time of bug-filing:
  https://chromeperf.appspot.com/group_report?sid=330d4707827868f45ace309394560cc4967231e05a4672a88f21c67816ede4f3


Bot(s) for this bug's original alert(s):

android-nexus5
android-nexus5X
android-one
android-webview-nexus6
chromium-rel-mac11-air

📍 Pinpoint job started.
https://pinpoint-dot-chromeperf.appspot.com/job/14d37c2a240000

📍 Couldn't reproduce a difference.
https://pinpoint-dot-chromeperf.appspot.com/job/14d37c2a240000

📍 Pinpoint job started.
https://pinpoint-dot-chromeperf.appspot.com/job/12b8aaaa240000

📍 Found a significant difference after 1 commit.
https://pinpoint-dot-chromeperf.appspot.com/job/12b8aaaa240000

Fix regression in thread_times.key_mobile_sites_smooth by yiyix@chromium.org
https://chromium.googlesource.com/chromium/src/+/be2578711ee47f8b8a5ba8b15108dedb50a27367

Understanding performance regressions:
  http://g.co/ChromePerformanceRegressions

the improvement was due to this cl, https://chromium-review.googlesource.com/c/chromium/src/+/1029733 and it has cause more 20 regression in other metrics. the cl mentioned here, has fixed those regressions and smoothness:sync_scroll.key_mobile_sites_smooth timing is back to the point before the fake improvement. No bug.