Mark external/wpt/dom/events/Event-dispatch-on-disabled-elements.html as flaky

The latest test in the file can fail at times.

TBR=tkent, hayato

Bug: 901271
Change-Id: Ifac53877b1a2da53464e12ce1222d6b15220b9a7
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/1313930Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#604871}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5516,4 +5516,7 @@ crbug.com/900356 [ Win ] virtual/threaded/animations/animationworklet/worklet-an
 crbug.com/900326 [ Win7 Mac10.12 ] http/tests/devtools/network/preview-searchable.js [ Timeout Pass ]
 crbug.com/900641 [ Linux ] compositing/transitions/transform-on-large-layer.html [ Timeout Pass ]
 crbug.com/900703 external/wpt/cors/origin.htm [ Failure Pass ]
-crbug.com/900730 [ Mac ] rootscroller/gesture-scroll-document-not-root-scroller.html [ Pass Failure Timeout Crash ]
\ No newline at end of file
+crbug.com/900730 [ Mac ] rootscroller/gesture-scroll-document-not-root-scroller.html [ Pass Failure Timeout Crash ]
+
+# ecosystem-infra sheriff 2018-11-02
+crbug.com/901271 external/wpt/dom/events/Event-dispatch-on-disabled-elements.html [ Pass Failure ]