Use getReferent() in enqueueSentinelReference to avoid race with GC.

This is the libcore part.

Fix a check failure due to the direct referent access that races with
the GC reference processing for CC.

Bug: 33207594
Bug: 12687968
Test: test-art-host with and without CC.

Change-Id: Ie1bda6e2fcafed2e6806e9244a7baa69d62022ad
1 file changed