Findit identified the culprit r576662 with confidence 100.0% in the config "chromium.chromiumos / linux-chromeos-rel"
based on the flakiness trend:

https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyxQELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKOAWNocm9taXVtLmNocm9taXVtb3MvbGludXgtY2hyb21lb3MtcmVsLzExNDA1L3Zpel9jb250ZW50X2Jyb3dzZXJ0ZXN0cy9VMmwwWlZCbGNsQnliMk5sYzNOSWFYUlVaWE4wUW5KdmQzTmxjbFJsYzNRdVVtOXZkRmRwYm1SdmQxUnlZVzV6Wm05eWJTOHcMCxITTWFzdGVyRmxha2VBbmFseXNpcxgCDA

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%20SitePerProcessHitTestBrowserTest.RootWindowTransform/0&comment=Link%20to%20Analysis%3A%20https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVyxQELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCKOAWNocm9taXVtLmNocm9taXVtb3MvbGludXgtY2hyb21lb3MtcmVsLzExNDA1L3Zpel9jb250ZW50X2Jyb3dzZXJ0ZXN0cy9VMmwwWlZCbGNsQnliMk5sYzNOSWFYUlVaWE4wUW5KdmQzTmxjbFJsYzNRdVVtOXZkRmRwYm1SdmQxUnlZVzV6Wm05eWJTOHcMCxITTWFzdGVyRmxha2VBbmFseXNpcxgCDA

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

Hi riajiang@, could you see if the July 19th change above could be responsible for the flakiness?

 Issue 871321  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ac13eb1b044a0b4a75fdcb2545d79156fe554f74

commit ac13eb1b044a0b4a75fdcb2545d79156fe554f74
Author: Rune Lillesveen <futhark@chromium.org>
Date: Tue Aug 07 14:14:25 2018

Disabled flaky browser test.

SitePerProcessHighDPIHitTestBrowserTest.RootWindowTransform/0 and
SitePerProcessHitTestBrowserTest.RootWindowTransform/0 are reported as
flaky. Disabling.

TBR=riajiang@chromium.org

Bug:  871321 , 870744
Change-Id: I4003753932b98f83e509dfb0bea5f69b03eb7d1e
Reviewed-on: https://chromium-review.googlesource.com/1164961
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581216}
[modify] https://crrev.com/ac13eb1b044a0b4a75fdcb2545d79156fe554f74/content/browser/site_per_process_hit_test_browsertest.cc

The test is disabled, nothing is left here for sheriffs.

https://chromium-review.googlesource.com/c/chromium/src/+/1172723 was landed to address some of the other flakes in SitePerProcessHitTestBrowserTest, I'll keep an eye on it to see if that can fix other flakes.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1094a6a2ba925f7e708251ddd182afc2ecb97274

commit 1094a6a2ba925f7e708251ddd182afc2ecb97274
Author: W. James MacLean <wjmaclean@chromium.org>
Date: Mon Sep 10 20:44:08 2018

Tentatively re-enable SitePerProcess*HitTestBrowserTest.RootWindowTransform.

A number of changes have landed recently that may eliminate the
flakieness seen in this test. This CL re-enables the test, but feel
free to revert this if the flakes continue.

Bug: 870744
Change-Id: I859f7170dbcdd334f3e872b117d48412721b7fa0
Reviewed-on: https://chromium-review.googlesource.com/1217089
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590050}
[modify] https://crrev.com/1094a6a2ba925f7e708251ddd182afc2ecb97274/content/browser/site_per_process_hit_test_browsertest.cc