Only Microtask::performCheckpoint() if recursionLevel is 0

Source/core/dom/Microtask.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 30 matching lines @@
 static MicrotaskQueue& microtaskQueue()
 {
     DEFINE_STATIC_LOCAL(MicrotaskQueue, microtaskQueue, ());
     return microtaskQueue;
 }
 
 void Microtask::performCheckpoint()
 {
     V8PerIsolateData* isolateData = V8PerIsolateData::current();
     ASSERT(isolateData);
+    ASSERT(!isolateData->recursionLevel());
+
     if (isolateData->performingMicrotaskCheckpoint())
         return;
     isolateData->setPerformingMicrotaskCheckpoint(true);
 
     while (!microtaskQueue().isEmpty()) {
         Vector<MicrotaskCallback> microtasks;
         microtasks.swap(microtaskQueue());
         for (size_t i = 0; i < microtasks.size(); ++i) {
             microtasks[i]();
         }
     }
 
     isolateData->setPerformingMicrotaskCheckpoint(false);
 }
 
 void Microtask::enqueueMicrotask(MicrotaskCallback callback)
 {
     microtaskQueue().append(callback);
 }
 
 } // namespace WebCore