Bug 1687391 - Avoid using a spinlock in ThreadSafeWeakReference, r=glandium,mccr8,Gankra,sg

This new approach to weak references is roughly modeled after the approach used
by Rust's Arc<T>, and uses an atomic compare-and-swap loop to perform weak to
strong reference upgrades. This approach ends up moving the strong reference
count out of the tracked object and into the weak reference object, as the
strong reference count atomic needs to outlife the object itself.

Rust's Arc Weak::upgrade implementation:
https://github.com/rust-lang/rust/blob/d98d2f57d9b98325ff075c343d2c7695b66dfa7d/library/alloc/src/sync.rs#L1806-L1837

Differential Revision: https://phabricator.services.mozilla.com/D102245