Increase mediasource-play per-test timeout

The per-test timeout defaults to 2 seconds in testharness.js.
Sometimes this test takes longer, leading to flaky results.
This change increases this test's timeout from the default 2 seconds
to 5 seconds, and removes the timeout expectation.

R=acolwell@chromium.org
TEST=timeout no longer repros locally on linux Debug across --iterations=10 --repeat-each=10
BUG=278400

Review URL: https://chromiumcodereview.appspot.com/23452002

git-svn-id: svn://svn.chromium.org/blink/trunk@156741 bbb929c8-8fbe-4397-9dbb-9b2b20218538