[Animation Worklet] Update test expectation for playback-rate.https.html

The test is not failing anymore. Update the test expectation to reflect
that. See:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=virtual%2Fthreaded%2Fexternal%2Fwpt%2Fanimation-worklet%2Fplayback-rate.https.html

Change-Id: Ibf9fa97ab202a9c34852a292bdeeb8a5eaf55ba0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582048Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653587}

third_party/blink/web_tests/TestExpectations

@@ -2866,7 +2866,7 @@ crbug.com/808834 [ Win ] external/wpt/css/css-pseudo/first-letter-001.html [ Fai
 crbug.com/723741 virtual/threaded/http/tests/devtools/tracing/idle-callback.js [ Failure Crash Pass Timeout ]
 
 # Sheriff 2019-03-08
-crbug.com/940050 virtual/threaded/external/wpt/animation-worklet/playback-rate.https.html [ Failure Timeout ]
+crbug.com/940050 virtual/threaded/external/wpt/animation-worklet/playback-rate.https.html [ Pass Timeout ]
 
 # Untriaged failures after https://crrev.com/c/543695/.
 # These need to be updated but appear not to be related to that change.