'virtual/threaded/compositing/visibility/visibility-simple-video-layer.html' is flaky.

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=virtual%2Fthreaded%2Fcompositing%2Fvisibility%2Fvisibility-simple-video-layer.html for details.

BUG=326524
TBR=vollick
NOTRY=true

Review URL: https://codereview.chromium.org/98403004

git-svn-id: svn://svn.chromium.org/blink/trunk@163348 bbb929c8-8fbe-4397-9dbb-9b2b20218538