RFR: 8251850: Simplify ResourceMark constructors using delegation

[Kim Barrett]

> On Aug 16, 2020, at 1:25 AM, Ioi Lam <ioi.lam at oracle.com> wrote:
> Since the new function current_thread_or_null() is used in only one place, I think it's better to get rid of it, and change this constructor as:
> 
>   explicit ResourceMark(ResourceArea* area)
>     : ResourceMark(area, DEBUG_ONLY(Thread::current_or_null()) NOT_DEBUG(nullptr)) {}

I’ll do that.

> PS, NOT_PRODUCT_ARG does come with a comma:
> 
> #define NOT_PRODUCT_ARG(arg) arg,

Oh, I’ve never had cause to use one of those.  I hardly ever write anything conditional on PRODUCT.