"page-screenshot.spec.ts - should work with a mobile viewport" is flaky on Chromium

[aslushnikov]

Test page-screenshot.spec.ts - should work with a mobile viewport is failing after the last roll. Requires investigation.

https://devops.aslushnikov.com/flakiness2.html#timestamp=1613768858686

[aslushnikov]

Upstream change that caused the regression: https://chromium-review.googlesource.com/c/chromium/src/+/2643792

(the failure was 100% reproducible on mac arm, so it was used to bisect the failure)

[aslushnikov]

@dgozman fixed this awhile ago!