[sheriff] Mark external/wpt/preload/download-resources.html as flaky.

TBR=yoavweiss@chromium.org
R=yoavweiss@chromium.org

Bug: 1164459
Change-Id: I6d70a3ab12f51b4b777de99693fdb0684ffe65d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627688Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843035}

third_party/blink/web_tests/TestExpectations

@@ -5837,3 +5837,6 @@ crbug.com/1164568 [ Mac ] external/wpt/html/cross-origin-opener-policy/reporting
 
 # Sheriff 2021-01-12
 crbug.com/1163793 http/tests/inspector-protocol/page/page-events-associated.js [ Pass Failure ]
+
+# Sheriff 2021-01-13
+crbug.com/1164459 [ Mac10.15 ] external/wpt/preload/download-resources.html [ Pass Failure ]