[Sheriff] Mark animation-state-changes-positive-playback-rate.html as flaky

TBR=kevers@chromium.org

Bug: 960944
Change-Id: I89ca42f4bca5935e9db9b7c0a68b92c05a364cf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980877Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727229}

third_party/blink/web_tests/TestExpectations

@@ -6144,3 +6144,4 @@ crbug.com/859169 http/tests/devtools/layers/layer-compositing-reasons.js [ Pass
 crbug.com/1036626 http/tests/devtools/tracing/tracing-record-input-events.js [ Pass Failure ]
 crbug.com/1036626 virtual/threaded/http/tests/devtools/tracing/tracing-record-input-events.js [ Pass Failure ]
 crbug.com/1036054 [ Mac10.13 ] virtual/threaded/fast/scroll-snap/snap-to-area-with-fractional-offset.html [ Pass Failure ]
+crbug.com/960944 animations/web-animations/animation-state-changes-positive-playback-rate.html [ Pass Failure ]