Visit class native roots from VisitReferences

Visit class roots when we call Class::VisitReferences instead of in
the class linker. This makes it easier to implement class unloading
since unmarked classes won't have their roots visited by the class
linker.

Bug: 22181835
Change-Id: I63f31e5ebef7b2a0b764b3ba3cb038b3f561b379
17 files changed