auto merge of #4777 : dmarcos/servo/issue4692, r=jdm

...id '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/cell.rs

@@ -41,6 +41,13 @@ impl<T> DOMRefCell<T> {
         &*self.value.as_unsafe_cell().get()
     }
 
+    /// Borrow the contents for the purpose of script deallocation.
+    ///
+    pub unsafe fn borrow_for_script_deallocation<'a>(&'a self) -> &'a mut T {
+        debug_assert!(task_state::get().contains(SCRIPT));
+        &mut *self.value.as_unsafe_cell().get()
+    }
+
     /// Is the cell mutably borrowed?
     ///
     /// For safety checks in debug builds only.

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);
-    }
-}