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

src/api.cc

Index: src/api.cc
===================================================================
--- src/api.cc	(revision 10391)
+++ src/api.cc	(working copy)
@@ -6029,7 +6029,25 @@
                                                              callback);
 }
 
+void HeapProfiler::VisitExternalResources(
+    v8::HeapProfiler::ExternalResourceVisitor* visitor) {
+  i::Isolate* isolate = i::Isolate::Current();
+  IsDeadCheck(isolate, "v8::HeapProfiler::VisitExternalResources");
 
+  class VisitorImpl : public i::HeapProfiler::ExternalResourceVisitor {
+   public:
+    VisitorImpl(v8::HeapProfiler::ExternalResourceVisitor* visitor)
+        : visitor_(visitor) {}
+    virtual ~VisitorImpl() {}
+    virtual void VisitExternalString(i::String* string) {
+      visitor_->VisitExternalString(Utils::ToLocal(
+          i::Handle<i::String>(string)));
+    }
+   private:
+    v8::HeapProfiler::ExternalResourceVisitor* visitor_;
+  } internal_visitor(visitor);
+  i::HeapProfiler::VisitExternalResources(&internal_visitor);
+}
 
 v8::Testing::StressType internal::Testing::stress_type_ =
     v8::Testing::kStressTypeOpt;