Findit identified the culprit r576290 with confidence 70.0% in the config "chromium.mac / Mac10.11 Tests"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy6QELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKyAWNocm9taXVtLm1hYy9NYWMxMC4xMSBUZXN0cy8yODMxOS93ZWJraXRfbGF5b3V0X3Rlc3RzIG9uIChub25lKSBHUFUgb24gTWFjIG9uIE1hYy0xMC4xMS42L1pYaDBaWEp1WVd3dmQzQjBMMk56Y3k5amMzTXRiR0Y1YjNWMExXRndhUzloZFhSdkxXSnNiMk5yTFhOcGVtVXRhVzVtYkc5M0xtaDBkSEJ6TG1oMGJXdz0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

If the culprit above is wrong, please file a bug using this link and hit submit:
https://bugs.chromium.org/p/chromium/issues/entry?status=Unconfirmed&labels=Pri-1,Test-Findit-Wrong&components=Tools%3ETest%3EFindit%3EFlakiness&summary=%5BFindit%5D%20Flake%20Analyzer%20-%20Wrong%20result%20for%20external/wpt/css/css-layout-api/auto-block-size-inflow.https.html&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVy6QELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKyAWNocm9taXVtLm1hYy9NYWMxMC4xMSBUZXN0cy8yODMxOS93ZWJraXRfbGF5b3V0X3Rlc3RzIG9uIChub25lKSBHUFUgb24gTWFjIG9uIE1hYy0xMC4xMS42L1pYaDBaWEp1WVd3dmQzQjBMMk56Y3k5amMzTXRiR0Y1YjNWMExXRndhUzloZFhSdkxXSnNiMk5yTFhOcGVtVXRhVzVtYkc5M0xtaDBkSEJ6TG1oMGJXdz0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Automatically posted by the findit-for-me app (https://goo.gl/Ot9f7N).

Hi Xiaocheng,

Can you take a look please?  Could your change below have caused this flake?  Thanks.

https://chromium.googlesource.com/chromium/src/+/08bb68ed2ad257016a3d69aa6075da1d6c11f481

My change only touches a test file, and can't possibly cause flakes in another one.

Hi tommycli@, this (bot-run) points to your r576290, any chance your change made these tests asynchronous? Hoping you have more context into this code than us sheriffs and can help triage. 

Hey pbos. It could be possible. Let me pass this to Fady and maybe he would have some insight.

Looks like the screenshots of the scrollbar can actually render correctly now (but perhaps racy since this doesn't fail 100%):

https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=ee553de65f7c399461fe4062ecfdbaca646150da&as=auto-block-size-inflow.https-actual.png

Expected has a black box instead:

https://isolateserver.appspot.com/browse?namespace=default-gzip&digest=21976d467c29ca1f858db7666bea98dbba99e098&as=auto-block-size-inflow.https-expected.png

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d83dede9e8d7ca534bd7d5342c00b4ad7093fe83

commit d83dede9e8d7ca534bd7d5342c00b4ad7093fe83
Author: Peter Boström <pbos@chromium.org>
Date: Tue Jul 31 02:10:05 2018

Disable auto-block-size-inflow.https.html

This test has started to flake recently, disabling during investigation.

Bug: chromium:868706
Tbr: tommycli@chromium.org
Change-Id: Icb9767c98885c76dece2f4b39449bf6e04a6ddcb
Reviewed-on: https://chromium-review.googlesource.com/1155993
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579278}
[modify] https://crrev.com/d83dede9e8d7ca534bd7d5342c00b4ad7093fe83/third_party/WebKit/LayoutTests/TestExpectations

Dropping the sheriff label as this is being investigated.

I really don't see how this patch can cause this flake. This is probably existing flake. Passing to chrishtr@ for triage to someone on the blink team.