RFR: Small cleanups in java.lang.ref

[mark.reinhold at oracle.com]

2018/5/16 18:31:15 -0700, martinrb at google.com:
> I've been confused by NULL vs null for years.

That’s because `NULL` in ReferenceQueue.java refers to the singleton
object `ReferenceQueue.NULL`, not the null value.  See the long comment
at the top of Reference.java for an explanation.

To improve this you could change mentions of `NULL` in the comments to
`ReferenceQueue.NULL`, but changing them to `null` would be incorrect.

> 8203327: Small cleanups in java.lang.ref
> http://cr.openjdk.java.net/~martin/webrevs/jdk/Reference-cleanup/
> https://bugs.openjdk.java.net/browse/JDK-8203327

The other changes look fine.

- Mark