I'm looking into this.

Found the cause - a racy navigation in JavaScript. Should be a straightforward fix.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c0d257e2981bd146615e72964db5d7d1ee4f4d51

commit c0d257e2981bd146615e72964db5d7d1ee4f4d51
Author: Dominick Ng <dominickn@chromium.org>
Date: Fri Feb 23 03:54:35 2018

Deflake AppBannerManagerBrowserTest.ExperimentalFlowWebAppBannerCancelled.

This test was flaky because the banner cancellation code also contained
a navigation in JavaScript to trigger the C++ callbacks for
cancellation. This navigation is necessary for some cancellation tests,
but not for others.

Deflake the test by removing the JavaScript navigation in this test,
where it isn't needed, and leave it in the tests which do need it.

BUG= 814928 

Change-Id: I6171da41e7c8a7cdc1cd5368371b1fbeeef14d61
Reviewed-on: https://chromium-review.googlesource.com/933561
Reviewed-by: Matt Giuca <mgiuca@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538691}
[modify] https://crrev.com/c0d257e2981bd146615e72964db5d7d1ee4f4d51/chrome/browser/banners/app_banner_manager_browsertest.cc
[modify] https://crrev.com/c0d257e2981bd146615e72964db5d7d1ee4f4d51/chrome/test/data/banners/main.js

Removing from the sheriff queue. Waiting for Dominick to mark as fixed.

No more flakes: https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=browser_tests&tests=AppBannerManagerBrowserTest.ExperimentalFlowWebAppBannerCancelled

Closing as Fixed.

Findit identified the culprit r538253 with confidence 100.0% in the config "chromium.memory / Linux Chromium OS ASan LSan Tests (1)"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy3QELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKmAWNocm9taXVtLm1lbW9yeS9MaW51eCBDaHJvbWl1bSBPUyBBU2FuIExTYW4gVGVzdHMgKDEpLzI2Mjg3L211c19icm93c2VyX3Rlc3RzL1FYQndRbUZ1Ym1WeVRXRnVZV2RsY2tKeWIzZHpaWEpVWlhOMExrVjRjR1Z5YVcxbGJuUmhiRVpzYjNkWFpXSkJjSEJDWVc1dVpYSkRZVzVqWld4c1pXUT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgCDA

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