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

This is the art 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: I3c98066d09dd98d8fddc59b6784c75843b45338b
1 file changed