Add a new test failure to wpt expectations

TBR=robertma

Change-Id: Ie2e46a32932b19544eb15d1b2f8080c75aab3e87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1994602Reviewed-by: Luke Z <lpz@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Luke Z <lpz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730181}

third_party/blink/web_tests/WPTOverrideExpectations

@@ -847,6 +847,7 @@ external/wpt/xhr/send-accept-language.htm [ Failure ]
 # Here are tests that were failing on WPT waterfall as of Dec. 2, 2019. They are triaged.
 # ---
 # Reftest with pixel diff. Fails locally. Fails on wpt.fyi
+external/wpt/compat/webkit-box-rtl-flex.html [ Failure ]
 external/wpt/css/css-fonts/font-display/font-display-change.html [ Failure ]
 external/wpt/css/css-layout-api/fallback-layout/constructor-error.https.html [ Failure ]
 external/wpt/mathml/presentation-markup/direction/direction-007.html [ Failure ]