Don't mutate the DOM from inside an Element's constructor

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 282 matching lines @@
 #endif
 {
     ASSERT(RuntimeEnabledFeatures::mediaEnabled());
 
     WTF_LOG(Media, "HTMLMediaElement::HTMLMediaElement");
     ScriptWrappable::init(this);
 
     if (document.settings() && document.settings()->mediaPlaybackRequiresUserGesture())
         m_userGestureRequiredForPlay = true;
 
-    // We must always have a ShadowRoot so children like <source> will not render
-    // as they never have an insertion point.
-    ensureUserAgentShadowRoot();
     setHasCustomStyleCallbacks();
     addElementToDocumentMap(this, &document);
 }
 
 HTMLMediaElement::~HTMLMediaElement()
 {
     WTF_LOG(Media, "HTMLMediaElement::~HTMLMediaElement");
 
     m_asyncEventQueue->close();
 
@@ skipping 3350 matching lines @@
     visitor->trace(m_error);
     visitor->trace(m_currentSourceNode);
     visitor->trace(m_nextChildNodeToConsider);
     visitor->trace(m_textTracks);
     visitor->trace(m_textTracksWhenResourceSelectionBegan);
     WillBeHeapSupplementable<HTMLMediaElement>::trace(visitor);
     HTMLElement::trace(visitor);
 }
 
 }