Sheriff: Mark animation-state-changes-negative-playback-rate.html with flaky

Bug: 986666
Change-Id: I0d45131ea62877e45b3151cf07d58c18f859282e
Notry: true
Tbr: nhiroki@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713150Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroki Nakagawa <nhiroki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679883}

third_party/blink/web_tests/TestExpectations

@@ -6329,3 +6329,7 @@ crbug.com/986477 [ Win ] virtual/samesite-by-default-cookies/external/wpt/cookie
 crbug.com/986019 animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]
 crbug.com/986019 virtual/threaded/animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]
 crbug.com/986019 virtual/disable-blink-gen-property-trees/animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]
+crbug.com/986666 animations/web-animations/animation-state-changes-negative-playback-rate.html [ Pass Failure ]
+crbug.com/986666 virtual/threaded/animations/web-animations/animation-state-changes-negative-playback-rate.html [ Pass Failure ]
+crbug.com/986666 virtual/disable-blink-gen-property-trees/animations/web-animations/animation-state-changes-negative-playback-rate.html [ Pass Failure ]
+