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/monitor.cc b/runtime/monitor.cc
index da6ee25..95fcb67 100644
--- a/runtime/monitor.cc
+++ b/runtime/monitor.cc
@@ -1126,24 +1126,19 @@
 }
 
 void MonitorList::DisallowNewMonitors() {
+  CHECK(!kUseReadBarrier);
   MutexLock mu(Thread::Current(), monitor_list_lock_);
   allow_new_monitors_ = false;
 }
 
 void MonitorList::AllowNewMonitors() {
+  CHECK(!kUseReadBarrier);
   Thread* self = Thread::Current();
   MutexLock mu(self, monitor_list_lock_);
   allow_new_monitors_ = true;
   monitor_add_condition_.Broadcast(self);
 }
 
-void MonitorList::EnsureNewMonitorsDisallowed() {
-  // Lock and unlock once to ensure that no threads are still in the
-  // middle of adding new monitors.
-  MutexLock mu(Thread::Current(), monitor_list_lock_);
-  CHECK(!allow_new_monitors_);
-}
-
 void MonitorList::BroadcastForNewMonitors() {
   CHECK(kUseReadBarrier);
   Thread* self = Thread::Current();