Auto merge of #18455 - nnethercote:measure-Arc-Locked-properly, r=jdm

Measure Arc<Locked<T>> fields properly.

Currently when we measure various Arc<Locked<T>> fields we don't measure the T
itself, but only the descendants of the T. This patch fixes this.

This fix requires introducing a new trait, MallocUnconditionalShallowSizeOf,
which is implemented for servo_arc::Arc. A similar trait,
MallocConditionalShallowSizeOf, is also introduced, though it has no uses as
yet.

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