Introduce MicrotasksCompletedCallback.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index a71dcfaec31a7725760646d1263013c3811f8d4d..7efc1a8d036a426ca711741bf57d339b091ad4e8 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7460,6 +7460,21 @@ bool Isolate::WillAutorunMicrotasks() const {
 }
 
 
+void Isolate::AddMicrotasksCompletedCallback(
+    MicrotasksCompletedCallback callback) {
+  if (callback == NULL) return;

[jochen (gone - plz use gerrit), 2016/02/25 12:00:39]

nullptr.

why should that be supported anyways? why not just DCHECK(callback)?

[dgozman, 2016/02/25 15:26:10]

I was following the AddCallCompletedCallback here thinking that's a common
practice in V8 api. Seems like it's not that common.

+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  isolate->AddMicrotasksCompletedCallback(callback);
+}
+
+
+void Isolate::RemoveMicrotasksCompletedCallback(
+    MicrotasksCompletedCallback callback) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  isolate->RemoveMicrotasksCompletedCallback(callback);
+}
+
+
 void Isolate::SetUseCounterCallback(UseCounterCallback callback) {
   reinterpret_cast<i::Isolate*>(this)->SetUseCounterCallback(callback);
 }