Relax CAS in some uses of Object::AtomicSetReadBarrierPointer.

Aside from the call to change the object from gray to black, the CAS in
AtomicSetReadBarrierPointer doesn't need to be a release CAS.

CC collector Ritz EAAC GC time: 28.4 -> 26.5s (-6.7%) on N5.

Bug: 12687968

Change-Id: Ic162717981b98e7f1e762fd6b27bd0c146187ca4
3 files changed