SPv2: Enable now-passing web-animations-api tests.

BUG=666986

Review-Url: https://codereview.chromium.org/2747083007
Cr-Commit-Position: refs/heads/master@{#457517}

third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2

@@ -70,7 +70,6 @@ Bug(none) typedcssom/ [ Skip ]
 Bug(none) usb/ [ Skip ]
 Bug(none) vibration/ [ Skip ]
 Bug(none) wake_lock/ [ Skip ]
-Bug(none) web-animations-api/ [ Skip ]
 Bug(none) webaudio/ [ Skip ]
 Bug(none) webexposed/ [ Skip ]
 Bug(none) webmidi/ [ Skip ]