Introduce debug events for promises.

src/debug.h

Index: src/debug.h
diff --git a/src/debug.h b/src/debug.h
index c295df2654bcf1b30c878d8a900386ba7becd430..cfd70590c27a49bb3da94cd48814e06a0b3c0956 100644
--- a/src/debug.h
+++ b/src/debug.h
@@ -365,6 +365,7 @@ class Debug {
   void OnCompileError(Handle<Script> script);
   void OnBeforeCompile(Handle<Script> script);
   void OnAfterCompile(Handle<Script> script);
+  void OnPromiseEvent(Handle<JSObject> data);
 
   // API facing.
   void SetEventListener(Handle<Object> callback, Handle<Object> data);
@@ -535,6 +536,8 @@ class Debug {
       Handle<Object> promise);
   MUST_USE_RESULT MaybeHandle<Object> MakeCompileEvent(
       Handle<Script> script, v8::DebugEvent type);
+  MUST_USE_RESULT MaybeHandle<Object> MakePromiseEvent(
+      Handle<JSObject> promise_event);
 
   // Mirror cache handling.
   void ClearMirrorCache();