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

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index b113d74792108c62ff13627b8117598c17b1b297..ec9a9846c11e9b2b78dcff938fbfa64497df3955 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -676,6 +676,11 @@ template <class T> class Persistent // NOLINT
   // 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
@@ -5349,6 +5354,22 @@ void Persistent<T>::SetWrapperClassId(uint16_t 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;