Add UseCounter for the Mozilla-style fullscreen error handling exemptions

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 156 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() && !inLegacyMozillaMode) {
+        if (!m_fullScreenElementStack.isEmpty()) {
             Element* lastElementOnStack = m_fullScreenElementStack.last().get();
-            if (lastElementOnStack == &element || !lastElementOnStack->contains(&element))
-                break;
+            if (lastElementOnStack == &element || !lastElementOnStack->contains(&element)) {
+                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()) {
             if (!descendant->isLocalFrame())
                 continue;
             ASSERT(toLocalFrame(descendant)->document());
             if (fullscreenElementFrom(*toLocalFrame(descendant)->document())) {
                 descendentHasNonEmptyStack = true;
                 break;
             }
         }
-        if (descendentHasNonEmptyStack && !inLegacyMozillaMode)
-            break;
+        if (descendentHasNonEmptyStack) {
+            if (inLegacyMozillaMode)
+                UseCounter::count(element.document(), UseCounter::LegacyFullScreenErrorExemption);
+            else
+                break;
+        }
 
         // This algorithm is not allowed to show a pop-up:
         //   An algorithm is allowed to show a pop-up if, in the task in which the algorithm is running, either:
         //   - an activation behavior is currently being processed whose click event was trusted, or
         //   - the event listener for a trusted click event is being handled.
         if (!UserGestureIndicator::processingUserGesture())
             break;
 
         // There is a previously-established user preference, security risk, or platform limitation.
 
@@ skipping 376 matching lines @@
 void FullscreenElementStack::trace(Visitor* visitor)
 {
     visitor->trace(m_fullScreenElement);
     visitor->trace(m_fullScreenElementStack);
     visitor->trace(m_fullScreenChangeEventTargetQueue);
     visitor->trace(m_fullScreenErrorEventTargetQueue);
     DocumentSupplement::trace(visitor);
 }
 
 } // namespace WebCore