8175085: [REDO] G1 Needs pre barrier on dereference of weak JNI handles

[Kim Barrett]

> On Feb 23, 2017, at 8:29 AM, Mikael Gerdin <mikael.gerdin at oracle.com> wrote:[…]
> 
> New webrevs:
> Incremental:
> http://cr.openjdk.java.net/~mgerdin/8175085/webrev.incremental/webrev/
> Full:
> http://cr.openjdk.java.net/~mgerdin/8175085/webrev.full2/webrev/

looks good