RFR: 8189633: Missing -Xcheck:jni checking for DeleteWeakGlobalRef

[Kim Barrett]

Please review this change to the checking form of DeleteWeakGlobalRef
to actually perform some checking.  The checking that is now being
done was cribbed from DeleteGlobalRef.

CR:
https://bugs.openjdk.java.net/browse/JDK-8189633

Webrev:
http://cr.openjdk.java.net/~kbarrett/8189633/open.00/

Testing:
Mach5 hs-tier1,2,3

I tried running tests locally with -Xcheck:jni, but that ran afoul of
JDK-8189766.