Add UseCounter for <iframe allowfullscreen> exemption for <video>

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 85f472d75043a36ad16d6f6c98ad334cfa966a02..6d7efb50345a1d6903cdb3969e2b95bd07cb443b 100644
--- a/Source/core/dom/FullscreenElementStack.cpp
+++ b/Source/core/dom/FullscreenElementStack.cpp
@@ -167,8 +167,12 @@ void FullscreenElementStack::requestFullScreenForElement(Element& element, Reque
 
         // 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;
+        if (!fullscreenIsAllowedForAllOwners(element.document())) {
+            if (requestType == PrefixedVideoRequest)
+                UseCounter::count(element.document(), UseCounter::VideoFullscreenAllowedExemption);
+            else
+                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