Bug 993772 - Stop serializing principals along with scripts. r=mrbkap

The principal of a script is determined by the compartment these days, so this
field is useless.
Bug 993772 - Introduce a singleton compilation scope. r=mrbkap
Bug 993772 - Switch to the singleton compilation scope for XBL compilation. r=mrbkap
Bug 993772 - Remove nsXBLDocGlobalObject. r=mrbkap
Bug 993772 - Switch to the singleton compilation scope for XUL prototype compilation. r=mrbkap
Bug 993772 - Remove nsXULPDGlobalObject. r=mrbkap