In bug 7183251 I found that we are caching in a map SunLayoutEngine instances
which really just wrap a copy of the key used to look them up.
Then SLE just calls a static native method.
If some native state/object were caching expensive state it might make
sense but setting up those native objects seems to be cheap.
It all seems like overhead. I think we can dispense with creating the map,
the keys and instances of SunLayoutEngine and just call the static native method.