[Sheriff] Disable document-user-activation.html on Linux.

It's flaky.

Tbr: tkent@chromium.org
Bug: 910211
Change-Id: I66a8cf70781d906fd97fda1a22f9d5050b273613
Reviewed-on: https://chromium-review.googlesource.com/c/1355487Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612255}

third_party/blink/web_tests/TestExpectations

@@ -5748,3 +5748,4 @@ crbug.com/908347 [ Mac ] media/autoplay/webaudio-audio-context-resume.html [ Fai
 
 # Sheriff 2018-11-29
 crbug.com/905772 [ Linux ] inspector-protocol/memory/sampling-native-profile-blink-gc.js [ Failure Pass ]
+crbug.com/910211 [ Linux ] virtual/video-surface-layer/media/autoplay/document-user-activation.html [ Failure Pass ]