Mark http/tests/media/media-source/webkitmediasource-seek-to-end-after-duration-change.html as timeout

http/tests/media/media-source/webkitmediasource-seek-to-end-after-
duration-change.html is already marked as Slow but is timing out ~10%
of the time. Mark as Pass Timeout.

TBR=acolwell@chromium.org
BUG=362507

Review URL: https://codereview.chromium.org/245403002

git-svn-id: svn://svn.chromium.org/blink/trunk@172045 bbb929c8-8fbe-4397-9dbb-9b2b20218538