All graphs for this bug:
  https://chromeperf.appspot.com/group_report?bug_id=814201

(For debugging:) Original alerts at time of bug-filing:
  https://chromeperf.appspot.com/group_report?sid=bef5be8962fc8e7d623c4fe2ba74f9ae40d40a056f902eecd1bd5f20f84b7e0d


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

chromium-rel-mac12

📍 Pinpoint job started.
https://pinpoint-dot-chromeperf.appspot.com/job/1596a3d7840000

📍 Couldn't reproduce a difference.
https://pinpoint-dot-chromeperf.appspot.com/job/1596a3d7840000

📍 Pinpoint job started.
https://pinpoint-dot-chromeperf.appspot.com/job/13852517840000

📍 Found significant differences after each of 4 commits.
https://chromeperf.appspot.com/job/13852517840000

[test_env.py] Warm up vpython virtualenv cache on swarming task shards. by iannucci@chromium.org
https://chromium.googlesource.com/chromium/src/+/8348d507e280f4bd02a7737bf5577f838d64437e

Revert "[test_env.py] Warm up vpython virtualenv cache on swarming task shards." by kbr@chromium.org
https://chromium.googlesource.com/chromium/src/+/7dde857919af2f59fcab10264d696301a15c64da

Add data received notification to WebContentsObserver. by chrisha@chromium.org
https://chromium.googlesource.com/chromium/src/+/f82f2a700cdcf2e62e0836f7de3706884fdab04c

[iOS] Disable a fullscreen egtest on devices. by liaoyuke@chromium.org
https://chromium.googlesource.com/chromium/src/+/312590315b04a872da752b1fda872d722590f584

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

Sorry I just saw this. The only thing I did was to disable a test on iOS, it couldn't be the culprit CL. chrisha@, can you take a look?

I added another method to WebContentsObserver that has no clients. It would be crazy for this to have caused the problem.

Looking at the performance graphs in question the problem appears to have gone away in early January, so was related to something else?