Mark document-user-activation-cross-origin-feature-policy-header.html as flaky

This test started to timeout on Linux:
https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVyfgsSBUZsYWtlInNjaHJvbWl1bUBibGlua193ZWJfdGVzdHNAaHR0cC90ZXN0cy9tZWRpYS9hdXRvcGxheS9kb2N1bWVudC11c2VyLWFjdGl2YXRpb24tY3Jvc3Mtb3JpZ2luLWZlYXR1cmUtcG9saWN5LWhlYWRlci5odG1sDA

TBR=beccahughes@chromium.org
NOTRY=True

Bug: 995663
Change-Id: I9a0d4bd2f63b1616b12999aaf7a500563401234f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2027901Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736445}

third_party/blink/web_tests/TestExpectations

@@ -6498,3 +6498,6 @@ crbug.com/1018655 external/wpt/html/cross-origin-opener-policy/blob-popup.https.
 
 # COOP random timeout failure.
 crbug.com/1046698 external/wpt/html/cross-origin-opener-policy/coep-blob-popup.https.html [ Timeout ]
+
+# Sheriff 2020-01-29
+crbug.com/995663 [ Linux ] http/tests/media/autoplay/document-user-activation-cross-origin-feature-policy-header.html [ Pass Failure Timeout ]