Mark download-resources.html as flaky.

R=wjmaclean

Bug: 1164459
Change-Id: Ic21cb372e8e8886075cbab00023dfe994a251d68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618525
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841620}

third_party/blink/web_tests/TestExpectations

@@ -5941,3 +5941,7 @@ crbug.com/1046784 http/tests/devtools/tracing/timeline-receive-response-event.js
 
 # Sheriff 2021-01-21
 crbug.com/1164166 [ Linux ] virtual/synchronous_html_parser/external/wpt/preload/avoid-prefetching-on-text-plain.html [ Pass Failure ]
+
+# Sheriff 2021-01-08
+crbug.com/1164459 [ Mac ] virtual/synchronous_html_parser/external/wpt/preload/download-resources.html [ Pass Timeout ]
+