Make it possible to disable read barriers in Class::GetObjectSize()

This is a leftover from cl/91831 (commit
9103c86a98524e9ddfd14f8cee56e919f68eee9b) that attempted to make it
possible to disable read barriers in Object::SizeOf().

Bug: 12687968
Change-Id: I2b05076832936881ec61bc21b6eb6b7c04e0a1f0
4 files changed