Propagate the read barrier option to Class::VisitNativeRoots.

Propagate the read barrier option from Object::VisitReferences to
Class::VisitNativeRoots.

This is a step toward the GC thread avoiding graying objects (and
reducing dirty pages) in the immune spaces.

Bug: 12687968

Change-Id: I29c4126a4ad4c40e63a934e62451fb3fb36aad43
9 files changed