Add a usecounter for a slotchange event listener

third_party/WebKit/Source/core/events/EventTarget.cpp

Index: third_party/WebKit/Source/core/events/EventTarget.cpp
diff --git a/third_party/WebKit/Source/core/events/EventTarget.cpp b/third_party/WebKit/Source/core/events/EventTarget.cpp
index e61dc73047a2f175133ab03bc672f27da73f22e8..cd8b30c521c9f4219c1ad85a184bb37cbd7d4c27 100644
--- a/third_party/WebKit/Source/core/events/EventTarget.cpp
+++ b/third_party/WebKit/Source/core/events/EventTarget.cpp
@@ -299,6 +299,10 @@ void EventTarget::addedEventListener(const AtomicString& eventType, RegisteredEv
         if (LocalDOMWindow* executingWindow = this->executingWindow()) {
             UseCounter::count(executingWindow->document(), UseCounter::PointerEventAddListenerCount);
         }
+    } else if (eventType == EventTypeNames::slotchange) {
+        if (LocalDOMWindow* executingWindow = this->executingWindow()) {
+            UseCounter::count(executingWindow->document(), UseCounter::SlotChangeEventAddListener);
+        }
     }
 }