Disabling flaky webkit test

TBR: pdr@chromium.org
Bug: 1046030
Change-Id: I0810c75544cec478ef2c71a7149a76a0cb99b487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023348Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735498}

third_party/blink/web_tests/FlagExpectations/enable-threaded-compositing

+# tags: [ Android Fuchsia IOS IOS12.2 IOS13.0 Linux Mac Mac10.10 Mac10.11 Mac10.12 Retina
+#         Mac10.13 Win Win7 Win10 ]
+# tags: [ Release Debug ]
+# results: [ Timeout Crash Pass Failure Slow Skip ]
+
+# Expectations for EnableThreadedCompositing
+[ Linux Debug ] virtual/threaded/external/wpt/web-animations/timing-model/animations/update-playback-rate-zero.html [ Failure ]