Suppress doubletap-to-jump-backwards-at-start.html failure on Linux

TBR=steimel@chromium.org

Bug: 831720
Change-Id: I22a9a233fd8442d29a849dd72c0fa75ae8dab30e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762928Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688763}

third_party/blink/web_tests/TestExpectations

@@ -5370,6 +5370,7 @@ crbug.com/831720 [ Win ] media/controls/tap-to-hide-controls.html [ Pass Failure
 crbug.com/831720 [ Linux ] media/controls/tap-to-hide-controls.html [ Pass Failure ]
 crbug.com/831720 [ Win ] virtual/audio-service/media/controls/doubletap-to-jump-forwards-too-short.html [ Pass Failure ]
 crbug.com/831720 [ Linux ] virtual/audio-service/media/controls/doubletap-to-jump-forwards-too-short.html [ Pass Failure ]
+crbug.com/831720 [ Linux ] virtual/audio-service/media/controls/doubletap-to-jump-backwards-at-start.html [ Pass Failure ]
 crbug.com/831720 [ Mac ] virtual/audio-service/media/controls/tap-to-hide-controls.html [ Pass Failure ]
 crbug.com/831720 [ Win ] virtual/audio-service/media/controls/tap-to-hide-controls.html [ Pass Failure ]
 crbug.com/831720 [ Linux ] virtual/audio-service/media/controls/tap-to-hide-controls.html [ Pass Failure ]