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

This reverts commit 9e3e507a.

Reason for revert: Disabled wrong test.

Original change's description:
> 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/997197
> Reviewed-by: Darren Shen <shend@chromium.org>
> Commit-Queue: Darren Shen <shend@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#548371}

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

Change-Id: I4000d62b9e4802214eabd139d62027d9e5664c50
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 829228
Reviewed-on: https://chromium-review.googlesource.com/999073
Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548606}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3804,7 +3804,6 @@ crbug.com/783154 [ Mac ] virtual/modern-media-controls/media/controls/modern/dou
 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/829228 [ Win7 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 ]