RFR: 8189359: Move native weak oops cleaning out of ReferenceProcessor

[Stefan Karlsson]

Hi all,

Please review this patch to move the JNI global weak handle processing 
out of the ReferenceProcessor into a new class, WeakProcessor, that will 
be used to gather processing and cleaning of "native weak" oops.

After this patch the ReferenceProcessor will only deal with the Java 
level java.lang.ref weak references.

http://cr.openjdk.java.net/~stefank/8189359/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8189359

Note this patch only moves the JNIHandles::weak_oops_do calls into the 
new WeakProcessor. A subsequent patch for JDK-8189359 will move the 
JvmtiExport::weak_oops_do from JNIHandleBlock into the WeakProcessor.

Future patches like JDK-8171119, for example, will be able to add it's 
set of native weak oops into the new WeakProcessor functions and won't 
have to duplicate the code for all GCs or add call inside the 
ReferenceProcessor.

Tested with JPRT.

Thanks,
StefanK