Mark media/controls/modern/doubletap-to-jump-forwards.html as flaky

TBR=bokan@chromium.org

No-Try: true
Bug: 937215
Change-Id: I24f1a7920284a1ed87255e1948088004a4d5c7f5
Reviewed-on: https://chromium-review.googlesource.com/c/1495522
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636755}

third_party/blink/web_tests/TestExpectations

@@ -5954,3 +5954,7 @@ crbug.com/915352 virtual/threaded/external/wpt/animation-worklet/worklet-animati
 
 crbug.com/915352 [ Mac10.11 ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline.https.html [ Failure ]
 crbug.com/915352 [ Mac10.11 ] virtual/threaded/external/wpt/animation-worklet/worklet-animation-with-scroll-timeline-and-display-none.https.html [ Failure ]
+
+# Sheriff 2019-03-01
+crbug.com/937215 [ Linux ] media/controls/modern/doubletap-to-jump-forwards.html [ Pass Failure ]
+crbug.com/937215 [ Linux ] virtual/video-surface-layer/media/controls/modern/doubletap-to-jump-forwards.html [ Pass Failure ]