Add Persistent<T>::Reset which disposes the handle and redirects it to point to another object.

include/v8.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 658 matching lines @@
 
   V8_INLINE(uint16_t WrapperClassId() const);
 
   /**
    * Returns the class ID previously assigned to this handle or 0 if no class ID
    * was previously assigned.
    */
   // TODO(dcarney): remove before cutover
   V8_INLINE(uint16_t WrapperClassId(Isolate* isolate) const);
 
+  /**
+   * Disposes the current contents of the handle and replaces it.
+   */
+  V8_INLINE(void Reset(Isolate* isolate, const Handle<T>& other));
+
 #ifndef V8_USE_UNSAFE_HANDLES
 
 #ifndef V8_ALLOW_ACCESS_TO_PERSISTENT_IMPLICIT
 
  private:
 #endif
   // TODO(dcarney): make unlinkable before cutover
   V8_INLINE(Persistent(const Persistent& that)) : val_(that.val_) {}
   // TODO(dcarney): make unlinkable before cutover
   V8_INLINE(Persistent& operator=(const Persistent& that)) {  // NOLINT
@@ skipping 4653 matching lines @@
                     true,
                     I::kNodeIsPartiallyDependentShift);
 }
 
 template <class T>
 void Persistent<T>::SetWrapperClassId(uint16_t class_id) {
   SetWrapperClassId(Isolate::GetCurrent(), class_id);
 }
 
 template <class T>
+void Persistent<T>::Reset(Isolate* isolate, const Handle<T>& other) {
+  Dispose(isolate);
+#ifdef V8_USE_UNSAFE_HANDLES
+  *this = *New(isolate, other);
+#else
+  if (other.IsEmpty()) {
+    this->val_ = NULL;
+    return;
+  }
+  internal::Object** p = reinterpret_cast<internal::Object**>(other.val_);
+  this->val_ = reinterpret_cast<T*>(
+      V8::GlobalizeReference(reinterpret_cast<internal::Isolate*>(isolate), p));
+#endif
+}
+
+template <class T>
 void Persistent<T>::SetWrapperClassId(Isolate* isolate, uint16_t class_id) {
   typedef internal::Internals I;
   if (this->IsEmpty()) return;
   if (!I::IsInitialized(isolate)) return;
   internal::Object** obj = reinterpret_cast<internal::Object**>(this->val_);
   uint8_t* addr = reinterpret_cast<uint8_t*>(obj) + I::kNodeClassIdOffset;
   *reinterpret_cast<uint16_t*>(addr) = class_id;
 }
 
 template <class T>
@@ skipping 550 matching lines @@
 
 
 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_