Request for review (s) - 7098155: Resize young gen at full collections for UseParallelGC

[Jon Masamitsu]

7098155: Resize young gen at full collections for UseParallelGC

This change adds the call to resize_young_gen() to resize the young 
generation
  after a full collection.

http://cr.openjdk.java.net/~jmasa/7098155/webrev.00/

This is the CR I inherited from Tao.  I changed his original patch
to remove the flag that originally guarded the change.  The flag
was put in because we originally thought this would go into
the jdk8 release rather late and considered it safer to guard
the change.

Jon