Mark pointerevent_touch-action-table-test_touch-manual.html as Flaky on Mac10.12

NOTRY=true
TBR=nzolghadr@chromium.org

Bug: 840792
Change-Id: I4032cd2ec67b7158ce650927f7d78594ce09c4b2
Reviewed-on: https://chromium-review.googlesource.com/1049972
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556782}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4660,3 +4660,6 @@ crbug.com/836783 fast/peerconnection/RTCRtpSender-setParameters.html [ Pass Time
 crbug.com/838254 virtual/video-surface-layer/media/webkit-media-controls-webkit-appearance.html [ Failure ]
 
 crbug.com/839332 [ Mac ] virtual/video-surface-layer/media/video-no-audio.html [ Pass Failure ]
+
+# Sheriff 2018-05-17
+crbug.com/840792 [ Mac10.12 ] external/wpt/pointerevents/pointerevent_touch-action-table-test_touch-manual.html [ Pass Timeout ]