[Sheriff] Mark test as flaky

Sample build:
https://ci.chromium.org/ui/p/chromium/builders/ci/Mac10.15%20Tests/7525/test-results?expected=false&exonerated=true&flaky=true

Bug: 1167210
Change-Id: If80065473d3cac08bce82562cc730944b1cd0c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633349
Auto-Submit: Yi Gu <yigu@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844131}

third_party/blink/web_tests/TestExpectations

@@ -5879,3 +5879,6 @@ crbug.com/1164459 [ Mac10.15 ] external/wpt/preload/download-resources.html [ Pa
 crbug.com/1147859 external/wpt/css/css-pseudo/highlight-painting-order.html [ Failure ]
 
 crbug.com/1060837 virtual/plz-dedicated-worker/external/wpt/html/cross-origin-embedder-policy/dedicated-worker.https.html [ Timeout ]
+
+# Sheriff 2021-01-15
+crbug.com/1167210 [ Mac ] virtual/forced-high-contrast-colors/fast/css/forced-colors-mode/forced-colors-mode-15.html [ Pass Failure ]
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