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}

third_party/blink/web_tests/TestExpectations

@@ -5772,3 +5772,4 @@ crbug.com/1015187 [ Linux Mac ] virtual/cross-origin-embedder-policy/external/wp
 crbug.com/996250 external/wpt/web-nfc/NFCReader_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 ]