WebUI: Disable flaky CrActionMenu autoRepositioning test on MacOS.

This is part of CrElementsFocusTest.CrActionMenu test, and seems to be
flaking on 'chromium/ci/Mac10.15 Tests' bots.

Bug: 329266310
Change-Id: Ib994ec2b71de12f0bb456cbd58ad263994837e2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5366688


Reviewed-by: John Lee <johntlee@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271722}