Some fixes for the CC collector.

- Remove a DCHECK in DisableMarkingCheckpoint, which caused
  occasional (false) failures.
- Check the thread-local GetWeakRefAccessEnabled in boxed lambdas weak
  access.
- Add missing BroadcastForNewAllocationRecords and
  BroadcastForNewWeakBoxedLambdas. The lack of the former caused
  occasional deadlocks in the ddmc test.
- Remove the 'ensure system weaks disallowed' calls, which weren't
  useful and dead.

Bug: 12687968
Change-Id: I33850c8d12e6e1a3aed1c2bb18eba263cbab76e8