Bug 930091 - Clone marionette args into the sandbox rather than using a COW. r=mdas

The current setup causes the arguments object (which is modified from the
sandbox) to be a COW, which causes problems with our new restrictions on
COWs. According to jgriffin, these are should be JSON-serializable, so
the clone should be fine.
Bug 930091 - Check objects at COW membranes. r=gabor,r=msucan