Disable external/wpt/webmessaging/broadcastchannel/basics.html.

The test is flaky on trybots, in particular on Fuchia x64, but on other
platforms as well.

Bug: 1154178
Change-Id: I8f3ef8b8961c4de209a76c9bacd838091e0e83fd
TBR: mek@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575035Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Balazs Engedy <engedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834680}

third_party/blink/web_tests/TestExpectations

@@ -5881,3 +5881,6 @@ crbug.com/1144171 http/tests/devtools/report-API-errors.js [ Skip ]
 
 # Sheriff 2020-12-07
 crbug.com/1156175 external/wpt/requestidlecallback/callback-iframe.html [ Pass Timeout ]
+
+# Sheriff 2020-12-08
+crbug.com/1154178 external/wpt/webmessaging/broadcastchannel/basics.html [ Pass Failure ]