Revert of Remove the leak expectation for...

Revert of Remove the leak expectation for web-animations-api/player-cancel-event.html since it seems to be wo… (patchset #1 id:1 of https://codereview.chromium.org/1766243002/ )

Reason for revert:
https://codereview.chromium.org/1750453002 was reverted and now the leak is happening again.

Original issue's description:
> Remove the leak expectation for web-animations-api/player-cancel-event.html.
>
> It seems to have been fixed by https://codereview.chromium.org/1750453002
>
> BUG=588598
>
> Committed: https://crrev.com/bc817fe1b14552590efd801e683686074e928570
> Cr-Commit-Position: refs/heads/master@{#379721}

TBR=alancutter@chromium.org,dsansome@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=588598

Review URL: https://codereview.chromium.org/1809783002

Cr-Commit-Position: refs/heads/master@{#381552}

third_party/WebKit/LayoutTests/LeakExpectations

@@ -91,6 +91,8 @@ crbug.com/482050 svg/zoom/text/zoom-foreignObject.svg [ Leak ]
 crbug.com/564571 virtual/threaded/animations/svg-length-unittype-crash.html [ Leak ]
 crbug.com/571534 virtual/threaded/animations/svg-element-css-animation-crash.html [ Leak ]
 
+crbug.com/588598 web-animations-api/player-cancel-event.html [ Leak ]
+
 # -----------------------------------------------------------------
 # Untriaged but known leaks of ActiveDOMObject (fast).
 # -----------------------------------------------------------------