mark test message-port-start-and-close-different-microtask.html as flaky

TBR: alexclarke
Bug: 784909
Change-Id: I2439160310386e88d2b91555bb605e5b8329f1c6
Reviewed-on: https://chromium-review.googlesource.com/768850Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516349}

third_party/WebKit/LayoutTests/TestExpectations

@@ -195,6 +195,9 @@ crbug.com/644433 virtual/gpu/fast/canvas/OffscreenCanvas-2d-pattern-in-worker.ht
 crbug.com/702006 paint/invalidation/compositing/scrolling-neg-z-index-descendants-should-cause-repaint.html [ Failure ]
 crbug.com/702006 virtual/spv175/paint/invalidation/compositing/scrolling-neg-z-index-descendants-should-cause-repaint.html [ Skip ]
 
+# Sheriff failures 2017-11-14
+crbug.com/784909  fast/events/message-port-start-and-close-different-microtask.html [ Failure Pass ]
+
 ########## Genuinely flaky ##########
 crbug.com/624233 virtual/gpu-rasterization/images/color-profile-background-clip-text.html [ Pass Failure ]