Fix a crash and possible memory leak in AutoplayUmaHelper

There's a crash in
AutoplayUmaHelper::onVisibilityChangedForVideoMutedPlayMethod().
It might be possible that the callback is called after the observer
is released. This CL adds additional nullcheck to avoid the crash.

This CL also fixes a possible memory leak by changing
wrapPersistent to wrapWeakPersistent.

BUG=638595

Review-Url: https://codereview.chromium.org/2249223003
Cr-Commit-Position: refs/heads/master@{#412574}

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

@@ -81,7 +81,7 @@ void AutoplayUmaHelper::didMoveToNewDocument(Document& oldDocument)
 
 void AutoplayUmaHelper::onVisibilityChangedForVideoMutedPlayMethod(bool isVisible)
 {
-    if (!isVisible)
+    if (!isVisible || !m_videoMutedPlayMethodVisibilityObserver)
         return;
 
     recordVideoAutoplayMutedPlayMethodBecomesVisibleUma(true);
@@ -115,7 +115,7 @@ void AutoplayUmaHelper::handlePlayingEvent()
 {
     if (m_source == AutoplaySource::Method && m_element->isHTMLVideoElement() && m_element->muted()) {
         if (!m_videoMutedPlayMethodVisibilityObserver) {
-            m_videoMutedPlayMethodVisibilityObserver = new ElementVisibilityObserver(m_element, WTF::bind(&AutoplayUmaHelper::onVisibilityChangedForVideoMutedPlayMethod, wrapPersistent(this)));
+            m_videoMutedPlayMethodVisibilityObserver = new ElementVisibilityObserver(m_element, WTF::bind(&AutoplayUmaHelper::onVisibilityChangedForVideoMutedPlayMethod, wrapWeakPersistent(this)));
             m_videoMutedPlayMethodVisibilityObserver->start();
             m_element->document().domWindow()->addEventListener(EventTypeNames::unload, this, false);
         }