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 10398)
+++ src/heap.cc	(working copy)
@@ -1354,6 +1354,28 @@
 }
 
 
+void Heap::VisitExternalResources(v8::ExternalResourceVisitor* visitor) {
+  AssertNoAllocation no_allocation;
+
+  class VisitorAdapter : public ObjectVisitor {
+   public:
+    explicit VisitorAdapter(v8::ExternalResourceVisitor* visitor)
+        : visitor_(visitor) {}
+    virtual void VisitPointers(Object** start, Object** end) {
+      for (Object** p = start; p < end; p++) {
+        if ((*p)->IsExternalString()) {
+          visitor_->VisitExternalString(Utils::ToLocal(
+              Handle<String>(String::cast(*p))));
+        }
+      }
+    }
+   private:
+    v8::ExternalResourceVisitor* visitor_;
+  } visitor_adapter(visitor);
+  external_string_table_.Iterate(&visitor_adapter);
+}
+
+
 class NewSpaceScavenger : public StaticNewSpaceVisitor<NewSpaceScavenger> {
  public:
   static inline void VisitPointer(Heap* heap, Object** p) {