Track external allocated memory for weak persistent handles.

runtime/vm/heap.h

Index: runtime/vm/heap.h
===================================================================
--- runtime/vm/heap.h	(revision 33396)
+++ runtime/vm/heap.h	(working copy)
@@ -102,6 +102,10 @@
     return 0;
   }
 
+  // Track external data.
+  void AllocateExternal(intptr_t size, Space space);
+  void FreeExternal(intptr_t size, Space space);
+
   // Heap contains the specified address.
   bool Contains(uword addr) const;
   bool NewContains(uword addr) const;
@@ -162,9 +166,10 @@
   // Print heap sizes.
   void PrintSizes() const;
 
-  // Return amount of memory used and capacity in a space.
+  // Return amount of memory used and capacity in a space, excluding external.
   intptr_t UsedInWords(Space space) const;
   intptr_t CapacityInWords(Space space) const;
+  intptr_t ExternalInWords(Space space) const;
   // Return the amount of GCing in microseconds.
   int64_t GCTimeInMicros(Space space) const;