Change RefCells in the DOM to a custom JSRefCell/DOMRefCell

[pcwalton]

This ref cell would be just like the built-in ref cell but it would do two things:

1. Calling any borrow/borrow-mut method would ensure this is not the layout thread in debug builds.
2. A new special borrow-in-layout method would ensure that this is the layout thread in debug builds.

This would help prevent problems like the one that broke the in-progress Rust upgrade and would make our code nicer as well.

[tetsuharuohzeki]

Does this import cell.rs from rust master and custom it?
Or we introduce a new sub-type object like this?:

struct DOMRefCell<T> {
  base: RefCell<T>,
  ...
}
impl<T> DOMRefCell<T> {
  ...
}

[jdm]

I think it makes sense to wrap the original type like your example.

[jdm]

To implement the safety assertions, let's add a local_data! in layout_interface that LayoutTask::new initializes. Borrow and borrow_in_layout can check it inside cfg!(debug) blocks, since I'm pretty sure task-local data is relatively slow right now.

[tetsuharuohzeki]

Once we'll replace all usage of RefCell in script to DOMRefCell for the above safety checking. I'll start to work it :)