Move microtask queueing logic from JavaScript to C++

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 9b58321ddbf615371d3ccec9b0177883399b1ab5..1e955a92c6a275a41a65c6fb52a3227ce2a7b1ad 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -349,7 +349,7 @@ typedef List<HeapObject*> DebugObjectCache;
   /* AstNode state. */                                                         \
   V(int, ast_node_id, 0)                                                       \
   V(unsigned, ast_node_count, 0)                                               \
-  V(bool, microtask_pending, false)                                            \
+  V(int, pending_microtask_count, 0)                                           \
   V(bool, autorun_microtasks, true)                                            \
   V(HStatistics*, hstatistics, NULL)                                           \
   V(HTracer*, htracer, NULL)                                                   \
@@ -1067,6 +1067,7 @@ class Isolate {
   void RemoveCallCompletedCallback(CallCompletedCallback callback);
   void FireCallCompletedCallback();
 
+  void EnqueueMicrotask(Handle<JSFunction> microtask);
   void RunMicrotasks();
 
  private: