Expect mediasource-seek-during-pending-seek pass

This test is no longer failing, so this change removes the
failure expectation. r155762 appears to have fixed the
failure.

R=acolwell@chromium.org
BUG=252865
TEST=100 repetitions of this test on local debug linux build all passed

Review URL: https://chromiumcodereview.appspot.com/23533006

git-svn-id: svn://svn.chromium.org/blink/trunk@157009 bbb929c8-8fbe-4397-9dbb-9b2b20218538