Disable video-poster.html on Mac bots

TBR=chrishtr@chromium.org

Bug: 905860, 900602
Change-Id: I5d2a01010b4424ffa4312dfaf01e3ca0af24d182
Reviewed-on: https://chromium-review.googlesource.com/c/1338570Reviewed-by: Ella Ge <eirage@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608606}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5278,3 +5278,4 @@ crbug.com/497522 external/wpt/css/filter-effects/backdrop-filter-paint-order.htm
 crbug.com/905694 [ Linux ] virtual/threaded/fast/scroll-behavior/smooth-scroll/scroll-during-selection.html [ Failure Pass ]
 crbug.com/905772 [ Linux ] virtual/sampling-heap-profiler/inspector-protocol/memory/sampling-native-profile-partition-alloc.js [ Failure Pass ]
 crbug.com/905827 [ Mac10.13 ] fast/dom/StyleSheet/stylesheet-move-between-documents-crash.html [ Failure Pass ]
+crbug.com/905860 [ Mac10.11 Mac10.13 Retina ] compositing/video/video-poster.html [ Failure Pass ]
\ No newline at end of file