make empty string returnable by ReturnValue

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index e99c6d9a98035d9b6abe7d26705c7ae1b02a280e..b7eb29489f845be536a9c4a7151b2e449a2fa046 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -2845,6 +2845,7 @@ class ReturnValue {
   // Fast JS primitive setters
   V8_INLINE(void SetNull());
   V8_INLINE(void SetUndefined());
+  V8_INLINE(void SetEmptyString());
   // Convenience getter for Isolate
   V8_INLINE(Isolate* GetIsolate());
 
@@ -5730,6 +5731,12 @@ void ReturnValue<T>::SetUndefined() {
 }
 
 template<typename T>
+void ReturnValue<T>::SetEmptyString() {
+  typedef internal::Internals I;
+  *value_ = *I::GetRoot(GetIsolate(), I::kEmptyStringRootIndex);
+}
+
+template<typename T>
 Isolate* ReturnValue<T>::GetIsolate() {
   // Isolate is always the pointer below the default value on the stack.
   return *reinterpret_cast<Isolate**>(&value_[-2]);