Add Persistent::Upcast.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 5526705c84269523ab7eafd144ab26dd12aa594e..b2203ec9ca1afacba24ce873ecfb619bdc2ed82a 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1469,6 +1469,8 @@ class V8EXPORT Value : public Data {
   bool Equals(Handle<Value> that) const;
   bool StrictEquals(Handle<Value> that) const;
 
+  template <class T> V8_INLINE(static Value* Cast(T* value));
+
  private:
   V8_INLINE(bool QuickIsUndefined() const);
   V8_INLINE(bool QuickIsNull() const);
@@ -5987,6 +5989,11 @@ bool Value::QuickIsString() const {
 }
 
 
+template <class T> Value* Value::Cast(T* value) {
+  return static_cast<Value*>(value);
+}
+
+
 Symbol* Symbol::Cast(v8::Value* value) {
 #ifdef V8_ENABLE_CHECKS
   CheckCast(value);