Treat background finalization as another GC task so it won't run in parallel with the marker.

runtime/vm/heap.h

Index: runtime/vm/heap.h
diff --git a/runtime/vm/heap.h b/runtime/vm/heap.h
index d90adc4673d7d54bec39dab2cafa191a397f84e4..764caabdc46593bb0e3eb9d5ba08f4b25d0dcd03 100644
--- a/runtime/vm/heap.h
+++ b/runtime/vm/heap.h
@@ -253,10 +253,6 @@ class Heap {
   Monitor* barrier() const { return barrier_; }
   Monitor* barrier_done() const { return barrier_done_; }
 
-  Monitor* finalization_tasks_lock() const { return finalization_tasks_lock_; }
-  intptr_t finalization_tasks() const { return finalization_tasks_; }
-  void set_finalization_tasks(intptr_t count) { finalization_tasks_ = count; }
-
   void SetupExternalPage(void* pointer, uword size, bool is_executable) {
     old_space_.SetupExternalPage(pointer, size, is_executable);
   }
@@ -350,9 +346,6 @@ class Heap {
   Monitor* barrier_;
   Monitor* barrier_done_;
 
-  Monitor* finalization_tasks_lock_;
-  intptr_t finalization_tasks_;
-
   // GC stats collection.
   GCStats stats_;