Bug 1023087 - [Window Management] Marionette test for copy-paste

https://bugzilla.mozilla.org/show_bug.cgi?id=1023087
For the consistency[1], please move this line out of the `prototype`.
[1] https://github.com/mozilla-b2g/gaia/blob/master/apps/system/test/marionette/lib/fakemediaapp.js#L11