Resolve applied-rule stylesheet identities through StyleSheetsActor and
include Firefox parentStyleSheet, line, and column fields on rule forms.
Firefox can then show matched rules as stylesheet rules and open the
corresponding CSS source location from the Rules panel.
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.
