RFR: 8201826: G1: Don't invoke WeakProcessor if mark stack has overflowed

[Kim Barrett]

Please review this change to G1 concurrent marking to not invoke the
WeakProcessor when the mark stack has overflowed, as the is-alive
closure is not accurate in that situation.

CR:
https://bugs.openjdk.java.net/browse/JDK-8201826

Webrev:
http://cr.openjdk.java.net/~kbarrett/8201826/open.00/

Testing:
{jdk,hs}-tier{1,2,3} on all Oracle supported platforms.