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

[Roman Kennke]

Looks good to me.

Thanks,
Roman

> 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
>