Make LOCAL_CONTEXT_KEY safe and non-leaky.

`LOCAL_CONTEXT_KEY` is currently a `Cell<*mut LocalLayoutContext>`. The use
of the raw pointer means that the `LocalLayoutContext` is not dropped when
the thread dies; this leaks FreeType instances and probably other
things. There are also some unsafe getter functions in `LayoutContext`
(`font_context`, `applicable_declarations_cache` and
`style_sharing_candidate_cache`) that @eddyb says involve undefined
behaviour.

This changeset changes `LOCAL_CONTEXT_KEY` to
`RefCell<Option<Rc<LocalLayoutContext>>>`. This fixes the leak and also
results in safe getters.

(Fixes #6282.)

components/layout/layout_task.rs

@@ -312,7 +312,6 @@ impl LayoutTask {
         let reporter_name = format!("layout-reporter-{}", id.0);
         mem_profiler_chan.send(mem::ProfilerMsg::RegisterReporter(reporter_name.clone(), reporter));
 
-
         // Create the channel on which new animations can be sent.
         let (new_animations_sender, new_animations_receiver) = channel();
         let (image_cache_sender, image_cache_receiver) = channel();