Mark virtual/video-surface-layer/media/controls/text-track-menu-pointer-selection.html slow on Mac

See https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Fvideo-surface-layer%2Fmedia%2Fcontrols%2Ftext-track-menu-pointer-selection.html

TBR=mlamouri@chromium.org

Bug: 876050
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ic267158a7452484098a039d5fd3da1fbf655d22d
Reviewed-on: https://chromium-review.googlesource.com/1183047Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584686}

third_party/WebKit/LayoutTests/SlowTests

@@ -405,6 +405,7 @@ crbug.com/864887 [ Mac ] fast/scroll-snap/snaps-after-scrollbar-scrolling.html [
 
 crbug.com/865262 [ Mac ] media/controls/text-track-menu-pointer-selection.html [ Slow ]
 crbug.com/865262 [ Mac ] virtual/new-remote-playback-pipeline/media/controls/text-track-menu-pointer-selection.html [ Slow ]
+crbug.com/876050 [ Mac ] virtual/video-surface-layer/media/controls/text-track-menu-pointer-selection.html [ Slow ]
 
 ### virtual/gpu/fast/canvas/ blending layout tests are slow
 crbug.com/866850 [ Linux Mac ] virtual/gpu/fast/canvas/canvas-blend-image.html [ Slow ]