Fix error when run layout test of mediasource-seek-during-pending-seek.html

Error Message "Harness Error. harness_status.status = 2 , harness_status.message = null"

BUG=

Review URL: https://chromiumcodereview.appspot.com/21156006

git-svn-id: svn://svn.chromium.org/blink/trunk@155762 bbb929c8-8fbe-4397-9dbb-9b2b20218538