Provide a way for iterating through all external strings referenced from the JS heap

src/heap.cc

Index: src/heap.cc
===================================================================
--- src/heap.cc	(revision 10391)
+++ src/heap.cc	(working copy)
@@ -1354,6 +1354,19 @@
 }
 
 
+void Heap::VisitExternalResources(v8::ExternalResourceVisitor* visitor) {
+  CollectAllGarbage(kMakeHeapIterableMask);
+  AssertNoAllocation no_allocation;
+  HeapIterator iterator;
+  for (HeapObject* obj = iterator.next(); obj != NULL; obj = iterator.next()) {
+    if (obj->IsExternalString()) {
+      visitor->VisitExternalString(Utils::ToLocal(
+          i::Handle<i::String>(String::cast(obj))));
+    }
+  }
+}
+
+
 class NewSpaceScavenger : public StaticNewSpaceVisitor<NewSpaceScavenger> {
  public:
   static inline void VisitPointer(Heap* heap, Object** p) {