Mark external/wpt/webrtc/RTCDTMFSender-ontonechange.https.html as failing.

Bug: 888634
Change-Id: I81df8a179f925e6dcb07bbdec118f407f3395567
No-Try: true
Tbr: jbroman@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1240520Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593605}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5207,3 +5207,7 @@ crbug.com/887659 animations/animationworklet/worklet-animation-local-time-undefi
 crbug.com/887659 animations/animationworklet/worklet-animation-set-keyframes.html [ Crash ]
 crbug.com/887659 animations/animationworklet/worklet-animation-set-timing.html [ Crash ]
 crbug.com/887659 animations/animationworklet/worklet-animation-style-update.html [ Failure ]
+
+# Sheriff 2018-09-24
+crbug.com/888634 external/wpt/webrtc/RTCDTMFSender-ontonechange.https.html [ Failure ]
+crbug.com/888634 virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/RTCDTMFSender-ontonechange.https.html [ Failure ]