Disable test doubletap-to-jump-forwards-too-short on Linux

Follow-up to https://chromium-review.googlesource.com/997197 where,
presumably, the wrong test was disabled.

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

Bug: 829228
Change-Id: Icd680733174a87d05172fe842b0dc0eed46aa80d
Reviewed-on: https://chromium-review.googlesource.com/997745
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548429}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3795,6 +3795,7 @@ crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/dou
 
 # 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/829228 [ Linux ] virtual/modern-media-controls/media/controls/modern/doubletap-to-jump-forwards-too-short.html [ Failure ]
 
 crbug.com/802915 css3/blending/isolation-should-include-non-local-background.html [ Failure ]