Disable a failing test.

BUG=832842
TBR=hbos@chromium.org
NOTRY=true

Change-Id: I5329fc3c5b0c1e1d330471ceab8edf66ce24dbad
Reviewed-on: https://chromium-review.googlesource.com/1012454
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550735}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4081,3 +4081,6 @@ crbug.com/832274 [ Linux ] fast/forms/search/search-appearance-basic.html [ Pass
 # Sheriff 2018-04-12
 crbug.com/818324 [ Linux ] external/wpt/html/syntax/parsing/html5lib_template.html?run_type=uri [ Pass Timeout ]
 crbug.com/831993 [ Linux ] virtual/gpu-rasterization/images/cross-fade-overflow-position.html [ Pass Timeout ]
+
+# Sheriff 2018-04-13
+crbug.com/832842 [ Win7 ] virtual/webrtc-wpt-unified-plan/external/wpt/webrtc/RTCDTMFSender-ontonechange.https.html [ Failure ]