Simplify an error condition in requestFullScreenForElement()

Source/core/dom/FullscreenElementStack.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 155 matching lines @@
             break;
 
         // The context object's node document, or an ancestor browsing context's document does not have
         // the fullscreen enabled flag set.
         if (requestType != PrefixedVideoRequest && !fullscreenIsAllowedForAllOwners(element.document()))
             break;
 
         // The context object's node document fullscreen element stack is not empty and its top element
         // is not an ancestor of the context object. (NOTE: Ignore this requirement if the request was
         // made via the legacy Mozilla-style API.)
-        if (!m_fullScreenElementStack.isEmpty()) {
-            Element* lastElementOnStack = m_fullScreenElementStack.last().get();
-            if (lastElementOnStack == &element || !lastElementOnStack->contains(&element)) {
+        if (Element* lastElementOnStack = fullscreenElement()) {
+            if (!element.isDescendantOf(lastElementOnStack)) {
                 if (inLegacyMozillaMode)
                     UseCounter::count(element.document(), UseCounter::LegacyFullScreenErrorExemption);
                 else
                     break;
             }
         }
 
         // A descendant browsing context's document has a non-empty fullscreen element stack.
         bool descendentHasNonEmptyStack = false;
         for (Frame* descendant = document()->frame() ? document()->frame()->tree().traverseNext() : 0; descendant; descendant = descendant->tree().traverseNext()) {
@@ skipping 380 matching lines @@
 
 void FullscreenElementStack::trace(Visitor* visitor)
 {
     visitor->trace(m_fullScreenElement);
     visitor->trace(m_fullScreenElementStack);
     visitor->trace(m_eventQueue);
     DocumentSupplement::trace(visitor);
 }
 
 } // namespace WebCore