Be more conservative about safety of dictionary and union values.

Mark dictionaries containing GC values as must_root, and wrap them in
RootedTraceableBox in automatically-generated APIs. To accommodate
union variants that are now flagged as unsafe, add RootedTraceableBox
to union variants that need to be rooted, rather than wrapping the
entire union value.

components/script/dom/bindings/trace.rs

@@ -745,6 +745,7 @@ impl<'a, T: JSTraceable + 'static> Drop for RootedTraceable<'a, T> {
 /// If you have GC things like *mut JSObject or JSVal, use rooted!.
 /// If you have an arbitrary number of DomObjects to root, use rooted_vec!.
 /// If you know what you're doing, use this.
+#[allow_unrooted_interior]
 pub struct RootedTraceableBox<T: 'static + JSTraceable> {
     ptr: *mut T,
 }