Disable test doubletap-to-jump-forwards-too-short on WebKit trusty debug

Likely failing due to incorrect rebaselining from:
https://chromium.googlesource.com/chromium/src/+/7a8765a7a7e0158e7f82681d06ce798bf8901328

TBR=beccahughes@chromium.org
TBR=thomasanderson@chromium.org

Bug: 829228
Change-Id: Icf2894aae0a4c0ed3baa85d82f7dd47417410d23
Reviewed-on: https://chromium-review.googlesource.com/997197Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548371}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3789,6 +3789,9 @@ crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/dou
 crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-on-play-button.html [ Skip ]
 crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/doubletap-to-toggle-fullscreen.html [ Skip ]
 
+# Test failing on WebKit Trusty
+crbug.com/829228 [ Linux Debug ] virtual/modern-media-controls/media/controls/modern/doubletap-to-jump-backwards.html [ Failure ]
+
 crbug.com/802915 css3/blending/isolation-should-include-non-local-background.html [ Failure ]
 
 crbug.com/807838 external/wpt/service-workers/service-worker/worker-in-sandboxed-iframe-by-csp-fetch-event.https.html [ Crash Pass ]