Introduce HasPendingMicrotasks API.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index a12216aefdf82f7c7995159412ff141a8365f7b9..51c4ba92e732dad14871d7602d33450b01658f5e 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -375,6 +375,7 @@ typedef List<HeapObject*> DebugObjectCache;
   /* Serializer state. */                                                      \
   V(ExternalReferenceTable*, external_reference_table, NULL)                   \
   V(int, pending_microtask_count, 0)                                           \
+  V(int, running_microtask_count, 0)                                           \
   V(bool, autorun_microtasks, true)                                            \
   V(HStatistics*, hstatistics, NULL)                                           \
   V(HStatistics*, tstatistics, NULL)                                           \
@@ -1101,6 +1102,7 @@ class Isolate {
 
   void EnqueueMicrotask(Handle<Object> microtask);
   void RunMicrotasks();
+  bool HasMoreMicrotasksToRun() const;
 
   void SetUseCounterCallback(v8::Isolate::UseCounterCallback callback);
   void CountUsage(v8::Isolate::UseCounterFeature feature);