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

include/v8.h

Index: include/v8.h
===================================================================
--- include/v8.h	(revision 10398)
+++ include/v8.h	(working copy)
@@ -2848,7 +2848,18 @@
  */
 typedef bool (*EntropySource)(unsigned char* buffer, size_t length);
 
+
 /**
+ * Interface for iterating though all external resources in the heap.
+ */
+class V8EXPORT ExternalResourceVisitor {  // NOLINT
+ public:
+  virtual ~ExternalResourceVisitor() {}
+  virtual void VisitExternalString(Handle<String> string) {}
+};
+
+
+/**
  * Container class for static utility functions.
  */
 class V8EXPORT V8 {
@@ -3204,6 +3215,13 @@
   static void GetHeapStatistics(HeapStatistics* heap_statistics);
 
   /**
+   * Iterates through all external resources referenced from current isolate
+   * heap. This method is not expected to be used except for debugging purposes
+   * and may be quite slow.
+   */
+  static void VisitExternalResources(ExternalResourceVisitor* visitor);
+
+  /**
    * Optional notification that the embedder is idle.
    * V8 uses the notification to reduce memory footprint.
    * This call can be used repeatedly if the embedder remains idle.