Mark external/wpt/longtask-timing/longtask-in-sibling-iframe.html flaky

Bug: 927866
Change-Id: I7659f8262d7292e372d90a47f91d7e0dcad3af20
Reviewed-on: https://chromium-review.googlesource.com/c/1450531Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628445}

third_party/blink/web_tests/TestExpectations

@@ -5975,6 +5975,8 @@ crbug.com/922951 virtual/video-surface-layer/media/controls/overflow-menu-hide-o
 crbug.com/922951 virtual/video-surface-layer/media/controls/overflow-menu-hide-on-click-panel.html [ Skip ]
 crbug.com/922951 virtual/video-surface-layer/media/controls/overflow-menu-toggle-class-for-animation.html [ Skip ]
 
+crbug.com/927866 external/wpt/longtask-timing/longtask-in-sibling-iframe.html [ Pass Failure ]
+
 # Race: The RTCIceConnectionState can become "connected" before getStats()
 # returns candidate-pair whose state is "succeeded", this sounds like a
 # contradiction.