[api] add API for Promise status and result.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index b57a9ca9acbc914163d0b7be90d6ae2e4d47205c..f75ff4fb86476f847f88e9ffad9e465fed95e604 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -3699,6 +3699,12 @@ class V8_EXPORT Function : public Object {
  */
 class V8_EXPORT Promise : public Object {
  public:
+  /**
+   * State of the promise. Each value corresponds to one of the possible values
+   * of the [[PromiseState]] field.
+   */
+  enum PromiseState { kPending, kFulfilled, kRejected };
+
   class V8_EXPORT Resolver : public Object {
    public:
     /**
@@ -3755,6 +3761,17 @@ class V8_EXPORT Promise : public Object {
    */
   bool HasHandler();
 
+  /**
+   * Returns the content of the [[PromiseResult]] field. The Promise must not
+   * be pending.
+   */
+  Local<Value> Result();
+
+  /**
+   * Returns the value of the [[PromiseState]] field.
+   */
+  PromiseState State();
+
   V8_INLINE static Promise* Cast(Value* obj);
 
  private: