[Sheriff] Disable ChromeVoxLiveRegionsUnitTest.ModifyStatusLiveRegion due to flakiness.

Culprit CL: https://chromium-review.googlesource.com/c/chromium/src/+/1361598

Bug: 912772
Tbr: danakj@chromium.org
Change-Id: I6262ca1623dc23e6004131fd60c86c62b13c593f
Reviewed-on: https://chromium-review.googlesource.com/c/1367928
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614767}