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.