servo/servo — pull request #3770 by saneyuki (2014-10-22 04:10:14+00:00)
Title: Introduce safe checking whether DOMRefCell<T>.borrow_for_layout() is called from layout task

### Abstract
This pull request introduce these things for #3050:

* dynamically check for `DOMRefCell.borrow_for_layout()` method is only called from layout task.
* dynamically check for `DOMRefCell.borrow***()` methods are NOT called from layout task.

These approach do check dynamically whether methods can get a task local data if we call them on debug build.

## Note
This should be merged into the tree after #3737 was merged.