Track external allocated memory for weak persistent handles.

runtime/vm/pages.h

Index: runtime/vm/pages.h
===================================================================
--- runtime/vm/pages.h	(revision 33396)
+++ runtime/vm/pages.h	(working copy)
@@ -188,6 +188,9 @@
 
   intptr_t UsedInWords() const { return used_in_words_; }
   intptr_t CapacityInWords() const { return capacity_in_words_; }
+  intptr_t ExternalInWords() const {
+    return external_in_words_;
+  }
 
   bool Contains(uword addr) const;
   bool Contains(uword addr, HeapPage::PageType type) const;
@@ -238,6 +241,9 @@
 
   void PrintToJSONObject(JSONObject* object);
 
+  void AllocateExternal(intptr_t size);
+  void FreeExternal(intptr_t size);
+
  private:
   // Ids for time and data records in Heap::GCStats.
   enum {
@@ -280,6 +286,7 @@
   intptr_t max_capacity_in_words_;
   intptr_t capacity_in_words_;
   intptr_t used_in_words_;
+  intptr_t external_in_words_;
 
   // Keep track whether a MarkSweep is currently running.
   bool sweeping_;