Add Value::Cast.

It's needed for upcasting Persistent<Object> to Persistent<Value> after
handlepocalypse (with Persistent::As or Persistent::Cast).

BUG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/16206014

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14995 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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);
@@ -5997,6 +5999,11 @@ 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);

test/cctest/test-api.cc

@@ -2898,6 +2898,24 @@ 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);
+#ifdef V8_USE_UNSAFE_HANDLES
+  v8::Persistent<Value> global_value =
+      v8::Persistent<Value>::Cast(global_string);
+#else
+  v8::Persistent<Value>& global_value =
+      v8::Persistent<Value>::Cast(global_string);
+#endif
+  CHECK(v8::Local<v8::Value>::New(isolate, 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"));