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

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 83bacb136ec5ad97acff8d7274ebc187fdfa6eb7..b9a74c7b4b4fdbf3d5cc734f46fa73f20dac24ce 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -35,8 +35,6 @@ Heap::Heap(Isolate* isolate,
       old_space_(this, max_old_gen_words, max_external_words),
       barrier_(new Monitor()),
       barrier_done_(new Monitor()),
-      finalization_tasks_lock_(new Monitor()),
-      finalization_tasks_(0),
       read_only_(false),
       gc_new_space_in_progress_(false),
       gc_old_space_in_progress_(false) {
@@ -53,7 +51,6 @@ Heap::Heap(Isolate* isolate,
 Heap::~Heap() {
   delete barrier_;
   delete barrier_done_;
-  delete finalization_tasks_lock_;
 
   for (int sel = 0;
        sel < kNumWeakSelectors;