Adds borrow_for_script_deallocation and unsafe_mut_js_info method to avoid 'DOMRefCell already mutably borrowed' messages. This is just a temporary fix until the Rust standard library allows borrowing already-borrowed RefCell values during unwinding.

It also removes LiveDOMReferences destructor that it's a no-op but it contains an assert that was being violated causing an endless cycle of destructor calls ending up in a stack overflow.

components/script/dom/bindings/refcounted.rs

@@ -188,9 +188,3 @@ impl LiveDOMReferences {
         })
     }
 }
-
-impl Drop for LiveDOMReferences {
-    fn drop(&mut self) {
-        assert!(self.table.borrow().keys().count() == 0);
-    }
-}