Make empty checkpoint work while weak ref access is disabled.

Fix a potential race on PushOntoMarkStack for CC by running an empty
checkpoint (while weak ref access is disabled).

Bug: 32508093
Bug: 12687968
Test: test-art-host with CC/CMS, libartd boot with N9, Ritz EAAC.
Change-Id: I3749bb525e7734804307ee16262355f3fc730312
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 043ff5d..02a6bb6 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -322,7 +322,10 @@
 
   void DisallowNewSystemWeaks() REQUIRES_SHARED(Locks::mutator_lock_);
   void AllowNewSystemWeaks() REQUIRES_SHARED(Locks::mutator_lock_);
-  void BroadcastForNewSystemWeaks() REQUIRES_SHARED(Locks::mutator_lock_);
+  // broadcast_for_checkpoint is true when we broadcast for making blocking threads to respond to
+  // checkpoint requests. It's false when we broadcast to unblock blocking threads after system weak
+  // access is reenabled.
+  void BroadcastForNewSystemWeaks(bool broadcast_for_checkpoint = false);
 
   // Visit all the roots. If only_dirty is true then non-dirty roots won't be visited. If
   // clean_dirty is true then dirty roots will be marked as non-dirty after visiting.