Mark virtual/off-main-thread-css-paint/.../paint2d-filter.https.html flaky

TBR=futhark@chromium.org

Bug: 1042205
Change-Id: I7b6e7ff570534f05b674dc2ecb4bbd7a67ff01e0
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006270Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732619}

third_party/blink/web_tests/TestExpectations

@@ -6399,5 +6399,6 @@ virtual/omt-worker-fetch/http/tests/origin_trials/sample-api-script-added-after-
 crbug.com/1042347 external/wpt/html/cross-origin-opener-policy/blob-popup.https.html [ Failure ]
 
 # Sheriff 2020-01-16
+crbug.com/1042205 virtual/off-main-thread-css-paint/external/wpt/css/css-paint-api/paint2d-filter.https.html [ Pass Timeout ]
 crbug.com/1042681 external/wpt/html/cross-origin-embedder-policy/blob.https.html [ Pass Failure ]
 crbug.com/1042694 fast/forms/form-control-with-state-eager-tracing-crashTest.html [ Pass Timeout ]