RFR: 8188066: (ref) Examine the reachability of JNI WeakGlobalRef and interaction with phantom refs

[Kim Barrett]

Please review this change to the JNI specification.  The specified
behavior of Weak Global References, and in particular their
relationship to Java PhantomReference, is being updated to account
for a change to PhantomReference by JDK-8071507.

CR:
https://bugs.openjdk.java.net/browse/JDK-8188066

CSR:
https://bugs.openjdk.java.net/browse/JDK-8220617

Changes:
http://cr.openjdk.java.net/~kbarrett/8188066/jni_specdiff_rev1.html
  specdiff for the changes.  In addition to updating the section
  describing Weak Global References, the description of NewGlobalRef
  is updated, as well as the copyright footer.

http://cr.openjdk.java.net/~kbarrett/8188066/weak_global_refs_rev1.md
  Copies of the before and after text for the JNI Weak Global
  References section; easier to read than diffs or specdiff.