Missing ScriptWrappable::init() in MediaKeyNeededEvent.

This path is previously uncovered by tests, since the test that ought to
cover it fails to trigger the event for some reason. So create one explicitly
in the test and add the initialization.

BUG=307069
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/26601009

git-svn-id: svn://svn.chromium.org/blink/trunk@159708 bbb929c8-8fbe-4397-9dbb-9b2b20218538