[api] Prototype WeakRef implementation

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index f1ba9a3347d7148cb7dfaced95d9a82fea412709..626cad87d6c684499b2c79a43166141b577c52c7 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -2166,6 +2166,11 @@ class V8_EXPORT Value : public Data {
   bool IsWeakSet() const;
 
   /**
+   * Returns true if this value is a WeakRef.
+   */
+  bool IsWeakRef() const;
+
+  /**
    * Returns true if this value is an ArrayBuffer.
    */
   bool IsArrayBuffer() const;