[Sheriff] Disabling blink flaky test: external/wpt/media-source/mediasource-detach.html

TBR=tkent

Bug: 1143720
Change-Id: I0dcd91b76d6c04e6aeea6ef3ed1faa89f29d969c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507542Reviewed-by: Parastoo Geranmayeh <parastoog@google.com>
Commit-Queue: Parastoo Geranmayeh <parastoog@google.com>
Auto-Submit: Parastoo Geranmayeh <parastoog@google.com>
Cr-Commit-Position: refs/heads/master@{#822238}

third_party/blink/web_tests/TestExpectations

@@ -6229,3 +6229,6 @@ crbug.com/1141206 scrollbars/overflow-scrollbar-combinations.html [ Pass Failure
 
 #Sheriff 2020-10-26
 crbug.com/1142877 external/wpt/mediacapture-fromelement/capture.html [ Pass Failure Crash ]
+
+#Sheriff 2020-10-29
+crbug.com/1143720 [ Win7 ] external/wpt/media-source/mediasource-detach.html [ Pass Failure ]
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