Do read barriers on native roots in CopyClassVisitor

Fixes a race condition caused by making a copy of a class then
clearing the native root arrays of the original class. If the
original class was gray and the new class was black then the GC
would miss the native roots.

Bug: 22957957

Change-Id: I706110018220af12a2ad0d72eb803f1cfe3580b9
2 files changed