Fix media-source LayoutTest timeouts caused by uncaught assert exceptions.

Some tests were timing out because AssertionError exceptions were not getting
caught because handleWaitCallback_() was not getting consistantly wrapped with
step_func(). This change ensures proper wrapping of the method and makes the
2 instances consistent.

BUG=284793

Review URL: https://codereview.chromium.org/67883005

git-svn-id: svn://svn.chromium.org/blink/trunk@162133 bbb929c8-8fbe-4397-9dbb-9b2b20218538