Add Persistent::Upcast.

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index cb3a38eee8e95f66dc294dce3dc5197050bc630e..d8760e8e8656c3669a4bfcf4523d82bc24aeab8f 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -2883,6 +2883,19 @@ THREADED_TEST(ClearAndLeakGlobal) {
 }
 
 
+THREADED_TEST(GlobalHandleUpcast) {
+  v8::Isolate* isolate = v8::Isolate::GetCurrent();
+  v8::HandleScope scope(isolate);
+  v8::Local<String> local = v8::Local<String>::New(v8_str("str"));
+  v8::Persistent<String> global_string(isolate, local);
+  v8::Persistent<Value>& global_value =
+      v8::Persistent<Value>::Upcast(global_string);
+  CHECK(global_value->IsString());
+  CHECK(global_string == v8::Persistent<String>::Cast(global_value));
+  global_string.Dispose();
+}
+
+
 THREADED_TEST(LocalHandle) {
   v8::HandleScope scope(v8::Isolate::GetCurrent());
   v8::Local<String> local = v8::Local<String>::New(v8_str("str"));