Mark fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html as flaky

This test is flaky under a variety of test suites:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyYwsSBUZsYWtlIlhjaHJvbWl1bUBibGlua193ZWJfdGVzdHNAZmFzdC9zY3JvbGxpbmcvc2Nyb2xsYmFycy9tb3VzZS1hdXRvc2Nyb2xsaW5nLW9uLXNjcm9sbGJhci5odG1sDA

TBR=arakeri@microsoft.com
NOTRY=TRUE

Bug: 1057060
Change-Id: Ib4fa290128f5b106c551bf756d0665d72117d112
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2230910Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775178}

third_party/blink/web_tests/TestExpectations

@@ -5958,4 +5958,5 @@ crbug.com/1083293 [ Linux ] media/video-aspect-ratio.html [ Pass Failure ]
 
 # Sheriff 2020-06-04
 crbug.com/1041973 virtual/web-components-v0-disabled/external/wpt/html/semantics/forms/constraints/form-validation-reportValidity.html [ Pass Failure Timeout ]
-crbug.com/1090087 transforms/2d/transform-2d.html [ Pass Timeout ]
\ No newline at end of file
+crbug.com/1090087 transforms/2d/transform-2d.html [ Pass Timeout ]
+crbug.com/1057060 fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Pass Failure Timeout ]