Add an API to control the disposal of external string resources...

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 4799)
+++ src/heap.h	(working copy)
@@ -690,6 +690,11 @@
   static bool GarbageCollectionGreedyCheck();
 #endif
 
+  static void SetExternalStringDiposeCallback(
+      ExternalStringDiposeCallback callback) {
+    external_string_dispose_callback_ = callback;
+  }
+
   static void AddGCPrologueCallback(
       GCEpilogueCallback callback, GCType gc_type_filter);
   static void RemoveGCPrologueCallback(GCEpilogueCallback callback);
@@ -1138,6 +1143,9 @@
   // any string when looked up in properties.
   static String* hidden_symbol_;
 
+  static ExternalStringDiposeCallback
+       external_string_dispose_callback_;
+
   // GC callback function, called before and after mark-compact GC.
   // Allocations in the callback function are disallowed.
   struct GCPrologueCallbackPair {