The PermGen Removal changed CompiledICHolders from being Java objects to C++ objects.

CodeCache::do_unloading(...) used to take a keep_alive closure that was applied to CompiledICHolders deep down in nmethod::do_unloading. Since CompileICHolders don't move anymore, and we have other ways to keep them alive, we don't use the keep_alive closures in these functions anymore.

Because of this, CodeCache::do_unloading will not populate the marking stacks, and we can remove the call to drain the stacks after the calls to CodeCache::do_unloading.

This also has the neat effect that we now can verify that the marking has really completed before we start unloading classes, which makes the code easier to maintain/understand.