These changes allow using MallocSizeOf/`#[conditional_malloc_size_of]`
on WebIDL callback values, and then fix a grab bag of places in the
script crate that previously ignored those values. There are also some
commits removing ignored fields that involved Arc/Rc that are not WebIDL
callbacks, since they are now easier to support with the
`#[conditional_malloc_size_of]` attribute.
Testing: Manual testing on about:memory for servo.org.
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Signed-off-by: Josh Matthews <josh@joshmatthews.net>