Mark an incoming web-animations test as flaky

Currently blocking the importer. Example:
https://test-results.appspot.com/data/layout_results/linux-rel/159555/not_site_per_process_webkit_layout_tests%20%28with%20patch%29/layout-test-results/results.html

No-Try because the change hasn't landed in Chromium yet.

TBR=flackr

No-Try: True
Bug: 991295
Change-Id: I599bd336d45df0ca928711657cb81cb838713990
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1738335Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684447}

third_party/blink/web_tests/TestExpectations

@@ -3292,6 +3292,9 @@ crbug.com/785230 external/wpt/css/css-text-decor/text-decoration-thickness-linet
 crbug.com/785230 external/wpt/css/css-text-decor/text-decoration-thickness-underline-001.html [ Failure ]
 crbug.com/785230 external/wpt/css/css-text-decor/text-decoration-thickness-vertical-002.html [ Failure ]
 
+# Flaky because of timestamps in error messages.
+crbug.com/991295 external/wpt/web-animations/timing-model/timelines/document-timelines.html [ Pass Failure ]
+
 # ====== New tests from wpt-importer added here ======
 crbug.com/626703 virtual/layout_ng_experimental/external/wpt/css/css-multicol/multicol-rule-nested-balancing-002.html [ Failure ]
 crbug.com/626703 external/wpt/css/css-multicol/multicol-breaking-000.html [ Failure ]