Mark video-controls-overflow-menu-last-button-visible flaky on Mac

See https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Mac10.11%20%28dbg%29

TBR=beccahughes@chromium.org,avayvod@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 789921
Change-Id: Ib4add80912d6ec2f95fe27224d06656702b7380f
Reviewed-on: https://chromium-review.googlesource.com/926363
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537772}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3137,7 +3137,7 @@ crbug.com/789478 virtual/prefer_compositing_to_lcd_text/scrollbars/custom-scroll
 crbug.com/789533 virtual/gpu/fast/canvas/OffscreenCanvas-2d-imageSmoothing.html [ Pass Failure Timeout ]
 
 # Sheriff failures 2017-11-30
-crbug.com/789921 [ Win7 ] media/video-controls-overflow-menu-last-button-visible.html [ Pass Failure ]
+crbug.com/789921 [ Win7 Mac ] media/video-controls-overflow-menu-last-button-visible.html [ Pass Failure ]
 crbug.com/789111 virtual/pwa-full-code-cache/http/tests/devtools/service-workers/service-worker-v8-cache.js [ Pass Failure Timeout ]
 crbug.com/790025 virtual/gpu-rasterization/images/image-hover-display-alt.html [ Failure Pass ]