Introduce v8::MicrotasksScope.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 09113c366d329bb28f92b62fc981193b2735cebb..bfa2216efb7da4cd1c600416d1f42efbcbf59a76 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2710,12 +2710,8 @@ void Isolate::RemoveCallCompletedCallback(CallCompletedCallback callback) {
 
 
 void Isolate::FireCallCompletedCallback() {
-  bool has_call_completed_callbacks = !call_completed_callbacks_.is_empty();
-  bool run_microtasks = autorun_microtasks() && pending_microtask_count();
-  if (!has_call_completed_callbacks && !run_microtasks) return;
-
-  if (!handle_scope_implementer()->CallDepthIsZero()) return;
-  if (run_microtasks) RunMicrotasks();
+  if (call_completed_callbacks_.is_empty()) return;
+  if (handle_scope_implementer()->GetCallDepth()) return;
   // Fire callbacks.  Increase call depth to prevent recursive callbacks.
   v8::Isolate* isolate = reinterpret_cast<v8::Isolate*>(this);
   v8::Isolate::SuppressMicrotaskExecutionScope suppress(isolate);