REGRESSION (r238090): [ MacOS WK1 ] Layout Test imported/blink/compositing/squashing/squashing-into-ancestor-painted-layer.html is flaky

[Truitt Savell]

The following layout test is flaky on MacOS WK1

imported/blink/compositing/squashing/squashing-into-ancestor-painted-layer.html

Probable cause:

This test has begun failing often recently on Debug WK1 and on other WK1 platforms less so. Failure comes from a square being to enlarged.

Flakiness Dashboard:

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=imported%2Fblink%2Fcompositing%2Fsquashing%2Fsquashing-into-ancestor-painted-layer.html

Image Diff:
https://build.webkit.org/results/Apple%20High%20Sierra%20Debug%20WK1%20(Tests)/r238171%20(6435)/imported/blink/compositing/squashing/squashing-into-ancestor-painted-layer-diffs.html

[Truitt Savell]

Reproduction:

run-webkit-tests --root debug-238090 imported/blink/compositing/squashing/squashing-into-ancestor-painted-layer.html --iterations 2000 -f --debug -1

I was able to reproduce this failure and bisect it to r238090. 

regression caused by https://trac.webkit.org/changeset/238090/webkit

The test does not fail on r238089

[Radar WebKit Bug Importer]

<rdar://problem/46066718>

[Ryan Haddad]

Marked the test as flaky in https://trac.webkit.org/r238707

[Alexey Proskuryakov]

This is still an issue.