This adds some overhead, but also provides the small performance benefit of
avoiding dirtying in the case where an event state is toggled an even
number of times between reflows.
The main benefit here though is that it sets us up to be smarter about
what we mark as dirty using restyle hints.
This method returned the result of `RefCell<T>.try_borrow().is_some()`.
But it meant whether the state **is not** BorrowState::Writing.
This wrong is introduced by [1], Before it revision, this method had
returned whether the state is BorrowState::Writing or not.
This doesn't change callers of this method because git blames ([3], [4]) says
that the caller is introduced by [2] and [2] is introduced before [1].
[1]: 86d609abaf
[2]: 49234484d6
[3]: 2cc08f289a/components/script/dom/servohtmlparser.rs
[4]: a7e29939a1/components/script/dom/bindings/cell.rs
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.
This used to conflict with the util crate from the standard library, which
has long since been removed.
The import in layout has not been changed because of a conflict with the
util mod there.
This gets rid of a dubious transmute:
let val = mem::transmute::<&RefCell<T>, &T>(&self.base);
The code duplication will be reduced once rust-lang/rust#18131 is fixed.
This type simply wraps `RefCell<T>` to add the special method,
and introduce the method to return the pointer of the value contained in
itself, for used in layout task.
