Mark mediasource-duration.html (wpt version) as flaky

Enabled in https://chromium-review.googlesource.com/599728 but now
failing on WebKit Linux Trusty (dbg) in the waterfall.

Put it with the same test in other places.

Bug: 689781
Change-Id: I37a8e13ae607a18fffe3b60aa4f8812e9aadd448
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/601911Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491994}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2718,6 +2718,7 @@ crbug.com/680050 inspector/sources/debugger-ui/watch-expressions-panel-switch.ht
 
 crbug.com/747751 http/tests/inspector/application-panel/resources-panel-resource-preview.html [ Failure Pass ]
 
+crbug.com/689781 external/wpt/media-source/mediasource-duration.html [ Failure Pass ]
 crbug.com/689781 http/tests/media/media-source/mediasource-duration.html [ Failure Pass ]
 crbug.com/689781 virtual/mojo-loading/http/tests/media/media-source/mediasource-duration.html [ Failure Pass ]