RFR: 8033923: Use BufferingOopClosure for G1 code root scanning

[Stefan Karlsson]

Hi all,

Please review this patch to use BufferingOopClosure for the code root 
scanning in G1.

Webrev:
  http://cr.openjdk.java.net/~stefank/8033923/webrev.00

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8033923

The patch builds upon the changes in:
  RFR: 8033764: Remove the usage of StarTask from BufferingOopClosure
  mail.openjdk.java.net/pipermail/hotspot-gc-dev/2014-February/009364.html

thanks,
StefanK