Add use counters for elements used in Shadow DOM

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

Index: third_party/WebKit/Source/core/html/HTMLContentElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLContentElement.cpp b/third_party/WebKit/Source/core/html/HTMLContentElement.cpp
index 623311092fb4e32800f2f9b2b70d18c672941fcb..765dd4221613762852561900280fd4b950ae0a78 100644
--- a/third_party/WebKit/Source/core/html/HTMLContentElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLContentElement.cpp
@@ -33,6 +33,7 @@
 #include "core/dom/shadow/ElementShadow.h"
 #include "core/dom/shadow/ElementShadowV0.h"
 #include "core/dom/shadow/ShadowRoot.h"
+#include "core/frame/UseCounter.h"
 #include "platform/RuntimeEnabledFeatures.h"
 
 namespace blink {
@@ -50,7 +51,9 @@ inline HTMLContentElement::HTMLContentElement(Document& document,
     : InsertionPoint(contentTag, document),
       m_shouldParseSelect(false),
       m_isValidSelector(true),
-      m_filter(filter) {}
+      m_filter(filter) {
+  UseCounter::count(document, UseCounter::HTMLContentElement);
+}
 
 HTMLContentElement::~HTMLContentElement() {}