[15] RFR 8242216: ObjectSampler::weak_oops_do() should not trigger barrier

[Zhengyu Gu]

Hi,

This is a similar problem as JDK-8237396.

Shenandoah does not expect barriers on it GC paths. Otherwise, it causes 
Shenandoah's OOM handler to fail.

Bug: https://bugs.openjdk.java.net/browse/JDK-8242216
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242216/webrev.00/index.html

Test:
   tier1 (fastdebug and release) on Linux x86_64
   Submit tests.


Thanks,

-Zhengyu