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

This reverts commit 4fd1b68c.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [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/1355487
> Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
> Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#612255}

TBR=sebsg@chromium.org

Change-Id: I9f41384c2a4b9dce100eecf1603f33a9f7ca6bbf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 910211
Reviewed-on: https://chromium-review.googlesource.com/c/1355431Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612276}

third_party/blink/web_tests/TestExpectations

@@ -5747,4 +5747,3 @@ 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 ]