Mark fast/forms/select/listbox-appearance-basic.html flaky on Windows

TBR=wangxianzhu@chromium.org

Bug: 1084509
Change-Id: If0ba708aeec31682f1852803930935ca24a5fddb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209672Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770352}

third_party/blink/web_tests/TestExpectations

@@ -5921,6 +5921,7 @@ crbug.com/1084276 [ Mac ] http/tests/security/offscreencanvas-placeholder-read-b
 
 # Sheriff 2020-05-19
 crbug.com/1084448 [ Win ] compositing/overflow/overflow-compositing-descendant.html [ Pass Failure ]
+crbug.com/1084509 [ Win ] fast/forms/select/listbox-appearance-basic.html [ Pass Failure ]
 
 # Disabled for landing DevTools change
 crbug.com/1011811 http/tests/devtools/network/network-xhr-data-received-async-response-type-blob.js [ Pass Failure ]