Do not create more than one ElementVisibilityObserver.

third_party/WebKit/Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1616 matching lines @@
         // Check for autoplay, and record metrics about it if needed.
         if (shouldAutoplay(RecordMetricsBehavior::DoRecord)) {
             recordAutoplaySourceMetric(AutoplaySourceAttribute);
 
             // If the autoplay experiment says that it's okay to play now,
             // then don't require a user gesture.
             m_autoplayHelper->becameReadyToPlay();
 
             if (!isGestureNeededForPlayback()) {
                 if (isHTMLVideoElement() && muted() && RuntimeEnabledFeatures::autoplayMutedVideosEnabled()) {
-                    m_autoplayVisibilityObserver = new ElementVisibilityObserver(this, WTF::bind(&HTMLMediaElement::onVisibilityChangedForAutoplay, wrapPersistent(this)));
-                    m_autoplayVisibilityObserver->start();
+                    // We might end up in a situation where the previous
+                    // observer didn't had time to fire yet. We can avoid
+                    // creating a new one in this case.
+                    if (!m_autoplayVisibilityObserver) {
+                        m_autoplayVisibilityObserver = new ElementVisibilityObserver(this, WTF::bind(&HTMLMediaElement::onVisibilityChangedForAutoplay, wrapPersistent(this)));
+                        m_autoplayVisibilityObserver->start();
+                    }
                 } else {
                     m_paused = false;
                     invalidateCachedTime();
                     scheduleEvent(EventTypeNames::play);
                     scheduleNotifyPlaying();
                     m_autoplaying = false;
                 }
             }
         }
 
@@ skipping 2379 matching lines @@
 
 IntRect HTMLMediaElement::AutoplayHelperClientImpl::absoluteBoundingBoxRect() const
 {
     IntRect result;
     if (LayoutObject* object = m_element->layoutObject())
         result = object->absoluteBoundingBoxRect();
     return result;
 }
 
 } // namespace blink