`ScavengeRootsTask` use `MarkingNMethodClosure` to process oops inside nmethods during young-gc. However, since young-gc using scavenging, instead of marking, there is no need for `MarkingNMethodClosure` -- `NMethodToOopClosure` is enough.

After switching to `NMethodToOopClosure`, the part of undoing the effect of nmethod-claiming inside `MarkingNMethodClosure` becomes unneeded, and `StrongRootsScope` becomes essential threads-claim-token only.

This is similar what's is done for Serial in JDK-8367417.