Track external allocated memory for weak persistent handles.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
===================================================================
--- runtime/vm/gc_marker.cc	(revision 33235)
+++ runtime/vm/gc_marker.cc	(working copy)
@@ -298,7 +298,7 @@
 
 class MarkingWeakVisitor : public HandleVisitor {
  public:
-  MarkingWeakVisitor() {
+  explicit MarkingWeakVisitor(Heap* heap) : heap_(heap) {
   }
 
   void VisitHandle(uword addr) {
@@ -306,11 +306,13 @@
         reinterpret_cast<FinalizablePersistentHandle*>(addr);
     RawObject* raw_obj = handle->raw();
     if (IsUnreachable(raw_obj)) {
-      FinalizablePersistentHandle::Finalize(handle);
+      handle->UpdateUnreachable(heap_);
     }
   }
 
  private:
+  Heap* heap_;
+
   DISALLOW_COPY_AND_ASSIGN(MarkingWeakVisitor);
 };
 
@@ -494,7 +496,7 @@
   IterateRoots(isolate, &mark, !invoke_api_callbacks);
   DrainMarkingStack(isolate, &mark);
   IterateWeakReferences(isolate, &mark);
-  MarkingWeakVisitor mark_weak;
+  MarkingWeakVisitor mark_weak(heap_);
   IterateWeakRoots(isolate, &mark_weak, invoke_api_callbacks);
   mark.Finalize();
   ProcessWeakTables(page_space);