We have a common pattern of creating a `WeakPtr<T>` from a reference and
passing that into a lambda, to then take the strong ref when the lambda
is executed. Add `weak_callback(Weakable, lambda)` that returns a lambda
that only invokes the callback if a strong ref exists, and passes it as
the first argument.
We were ignoring the dom_element_unique_id field, which meant if we had
more than one inline style sheet on a page, we would not distinguish
between them. This had the effect of halting the style sheet loading
process in the dev tools, with none appearing.
