Add use counters for elements used in Shadow DOM

third_party/WebKit/Source/core/html/HTMLSlotElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
index 97a1d1d768a22df09501df27567a3c6ea515a03e..46492d7e6ef33eeb2debc22d6348266c819c0e19 100644
--- a/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLSlotElement.cpp
@@ -39,6 +39,7 @@
 #include "core/dom/shadow/InsertionPoint.h"
 #include "core/dom/shadow/SlotAssignment.h"
 #include "core/events/Event.h"
+#include "core/frame/UseCounter.h"
 #include "core/html/AssignedNodesOptions.h"
 #include "core/probe/CoreProbes.h"
 
@@ -48,6 +49,7 @@ using namespace HTMLNames;
 
 inline HTMLSlotElement::HTMLSlotElement(Document& document)
     : HTMLElement(slotTag, document) {
+  UseCounter::count(document, UseCounter::HTMLSlotElement);
   setHasCustomStyleCallbacks();
 }