Let script collection hold a reference to the adapter

While the script collection always follows a page, it is safer to have
both hold a reference to the web-contents adapter, so it doesn't get
deleted while adapters are being changed.

Change-Id: I21e268f7228f13702468df61e0032bdf2f99873a
Reviewed-by: Michael Brüning <michael.bruning@qt.io>