Simplify an error condition in requestFullScreenForElement()

Source/core/dom/FullscreenElementStack.cpp

Index: Source/core/dom/FullscreenElementStack.cpp
diff --git a/Source/core/dom/FullscreenElementStack.cpp b/Source/core/dom/FullscreenElementStack.cpp
index c58d1f1b78b25470b5ee08fd04439cefba01f83b..85f472d75043a36ad16d6f6c98ad334cfa966a02 100644
--- a/Source/core/dom/FullscreenElementStack.cpp
+++ b/Source/core/dom/FullscreenElementStack.cpp
@@ -173,9 +173,8 @@ void FullscreenElementStack::requestFullScreenForElement(Element& element, Reque
         // 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