Replace &JSRef with JSRef in the bulk of the generated code. This will
remove a level of indirection throughout all DOM code.
This patch doesn't change methods implemented on JSRef<T> to take `self`
rather than `&self`, and it leaves a few other uses of &JSRef, but those
changes can be made incrementally.
Moved the Atob and Btoa methods ouside the impl for WorkerMethod trait
and made them publicly accessible from WorkerGlobalScopeMethods via
proxy methods.
