Mark web test as flaky

TBR=arakeri@microsoft.com

No-Try: true
Bug: 1058244
Change-Id: Ib1e272813e477245a98901ce9d6fe611616a252e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2092876Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747956}

third_party/blink/web_tests/TestExpectations

@@ -6767,3 +6767,4 @@ crbug.com/1059322 [ Mac ] fast/forms/file/file-input-webkitdirectory-key-space.h
 crbug.com/1059322 [ Mac ] fast/forms/file/file-input-click.html [ Pass Failure ]
 crbug.com/1059322 [ Mac ] fast/forms/autofocus-focus-only-once.html [ Pass Failure ]
 crbug.com/1059322 [ Mac ] virtual/form-controls-refresh-disabled/fast/forms/autofocus-focus-only-once.html [ Pass Failure ]
+crbug.com/1058244 [ Mac ] virtual/threaded-prefer-compositing/fast/scrolling/scrollbars/scrollbar-rtl-manipulation.html [ Pass Failure ]