Using a Promise in BackgroundAction was not doing anything since the
change to use a weak reference to the event loop, so let's just drop
that.
The thread will now always move itself (and therefore its callbacks)
over to the originating thread before completing, regardless of the
presence of callbacks. This ensures that ref counting remains on the
main thread.
In addition, BackgroundAction's completion callback can no longer
return errors. This functionality wasn't actually used anywhere, it was
a holdover from the behavior of Core::Promise.
When a BackgroundAction completes, it resolves a Promise (stored on the
BackgroundAction object) with a reference to itself. The Promise will
never unset this resolved value, thus it will hold a strong reference to
the BackgroundAction until it is destroyed. But because the Promise is
owned by the BackgroundAction itself, we have a reference cycle, and
neither object can be destroyed.
The only user of BackgroundAction is the ImageDecoder process. The
consequence was that the ImageDecoder process would never release any
image data for successfully decoded images.
To fix this, instead of storing the promise on the class itself, we can
just create it as a local variable and pass it around.
