Add DCHECK that Allocation PreFenceVisitors do not suspend

As PreFenceVisitors perform initial setup of objects before they are
fully visible to the GC they should not ever suspend. This adds a
ScopedAssertNoThreadSuspension around calls to the fence in order to
verify (on kIsDebugBuild) that no suspend occurs.

Test: ./test.py --host
Change-Id: I3b073252cf74289da30e1a5ebf4272dcc4c0023f
1 file changed