Do not create more than one ElementVisibilityObserver.

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

Index: third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
index 3983e1b82f1f7f8ba5754617a52b40633a1c087c..5a63b92a411057bd6934052df59836e6eadb1078 100644
--- a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
@@ -1634,8 +1634,13 @@ void HTMLMediaElement::setReadyState(ReadyState state)
 
             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();