Introduce MicrotasksCompletedCallback.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index a71dcfaec31a7725760646d1263013c3811f8d4d..43fbf9d8e7292ead079569109e7a9a04a116aa4e 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7460,6 +7460,21 @@ bool Isolate::WillAutorunMicrotasks() const {
 }
 
 
+void Isolate::AddMicrotasksCompletedCallback(
+    MicrotasksCompletedCallback callback) {
+  DCHECK(callback);
+  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);
 }