Introduce MicrotasksCompletedCallback.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 2d74dc4a63a9b89d4542eb1ec8a8ab24662ddd4d..25a3b2fe54bded3d4c71833157d4b421458699f2 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1061,6 +1061,10 @@ class Isolate {
   void RemoveBeforeCallEnteredCallback(BeforeCallEnteredCallback callback);
   void FireBeforeCallEnteredCallback();
 
+  void AddMicrotasksCompletedCallback(MicrotasksCompletedCallback callback);
+  void RemoveMicrotasksCompletedCallback(MicrotasksCompletedCallback callback);
+  void FireMicrotasksCompletedCallback();
+
   void SetPromiseRejectCallback(PromiseRejectCallback callback);
   void ReportPromiseReject(Handle<JSObject> promise, Handle<Object> value,
                            v8::PromiseRejectEvent event);
@@ -1209,6 +1213,8 @@ class Isolate {
   // the frame.
   void RemoveMaterializedObjectsOnUnwind(StackFrame* frame);
 
+  void RunMicrotasksInternal();
+
   base::Atomic32 id_;
   EntryStackItem* entry_stack_;
   int stack_trace_nesting_level_;
@@ -1330,6 +1336,9 @@ class Isolate {
   // List of callbacks when a Call completes.
   List<CallCompletedCallback> call_completed_callbacks_;
 
+  // List of callbacks after microtasks were run.
+  List<MicrotasksCompletedCallback> microtasks_completed_callbacks_;
+
   v8::Isolate::UseCounterCallback use_counter_callback_;
   BasicBlockProfiler* basic_block_profiler_;