RFR (S): 8213142: Use RAII to set the scanning source in G1ScanEvacuatedObjClosure

[Thomas Schatzl]

Hi all,

  in the review for JDK-8212911 Kim suggested to use RAII to set the
_scanning_in_young field of the G1ScanEvacuatedObjClosure class. This
also allows making sure that the field is actually initialized when
used.

This is the implementation for this idea.

CR:
https://bugs.openjdk.java.net/browse/JDK-8213142
Webrev:
http://cr.openjdk.java.net/~tschatzl/8213142/webrev/
Testing:
local gc jtreg run

Thanks,
  Thomas