Revert "Disable test compositing/video/video-poster.html"

This reverts commit 093809a5.

Reason for revert: The test has been all green since crrev.com/707576:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=compositing/video/video-poster.html

Original change's description:
> Disable test compositing/video/video-poster.html
> 
> compositing/video/video-poster.html
> is flaky from 10/18 on Windows, Mac and Linux.
> https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyUgsSBUZsYWtlIkdjaHJvbWl1bUBibGlua193ZWJfdGVzdHNfZXhwYXJjaGl2ZUBjb21wb3NpdGluZy92aWRlby92aWRlby1wb3N0ZXIuaHRtbAw
> 
> TBR=wangxianzhu@chromium.org
> 
> Bug: 1015764
> Change-Id: I3701bb7ea8cdacd3324469ab779239c80c903545
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1868981
> Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
> Reviewed-by: Lan Wei <lanwei@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#707494}

TBR=wangxianzhu@chromium.org,lanwei@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1015764
Change-Id: Ic02c83b3c8c689e580b9a9b7af7d3a9067fd715b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872454Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707830}

third_party/blink/web_tests/TestExpectations

@@ -5763,7 +5763,6 @@ crbug.com/1015187 [ Linux Mac ] virtual/cross-origin-embedder-policy/external/wp
 crbug.com/996250 external/wpt/web-nfc/NDEFReader_scan_iframe.https.html [ Timeout ]
 crbug.com/1015254 [ Mac ] external/wpt/pointerevents/extension/pointerevent_coalesced_events_attributes.html [ Pass Failure ]
 crbug.com/1015859 [ Linux ] http/tests/devtools/a11y-axe-core/performance/landing-page-a11y-test.js [ Pass Failure ]
-crbug.com/1015764 compositing/video/video-poster.html [ Pass Crash ]
 crbug.com/1015975 media/video-currentTime.html [ Pass Failure ]
 crbug.com/1015975 virtual/audio-service/media/video-currentTime.html [ Pass Failure ]