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
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index a5bc60a..57af959 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -499,7 +499,8 @@
     DCHECK(thread == self || thread->IsSuspended() || thread->GetState() == kWaitingPerformingGc)
         << thread->GetState() << " thread " << thread << " self " << self;
     // Disable the thread-local is_gc_marking flag.
-    DCHECK(thread->GetIsGcMarking());
+    // Note a thread that has just started right before this checkpoint may have already this flag
+    // set to false, which is ok.
     thread->SetIsGcMarking(false);
     // If thread is a running mutator, then act on behalf of the garbage collector.
     // See the code in ThreadList::RunCheckpoint.