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This commit adds `ThreadPtr{Handle, Holder}` to wrap an `XpCom` object
with thread-safe refcounting. These are analagous to
`nsMainThreadPtr{Handle, Holder}`, but can hold references to
objects from any thread, not just the main thread.
`ThreadPtrHolder` is similar to `ThreadBoundRefPtr`. However, it's
not possible to clone a `ThreadBoundRefPtr`, so it can't be shared
among tasks. This is fine for objects that are only used once, like
callbacks. However, `ThreadBoundRefPtr` doesn't work well for loggers
or event emitters, which might need to be called multiple times on
the owning thread.
Unlike a `ThreadBoundRefPtr`, it's allowed and expected to
clone and drop a `ThreadPtrHolder` on other threads. Internally,
the holder keeps an atomic refcount, and releases the wrapped object
on the owning thread once the count reaches zero.
This commit also changes `TaskRunnable` to support dispatching from
threads other than the main thread.
Differential Revision: https://phabricator.services.mozilla.com/D20074
--HG--
extra : moz-landing-system : lando