Add detailed use counters for document.registerElement use.

third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp

Index: third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp
diff --git a/third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp b/third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp
index e6cee541d9077db25e78785bf9401d68c4117e26..c1abb788028e24de5f934b9a31ebff2cb36d5de5 100644
--- a/third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp
+++ b/third_party/WebKit/Source/core/dom/custom/V0CustomElement.cpp
@@ -38,6 +38,7 @@
 #include "core/dom/custom/V0CustomElementMicrotaskRunQueue.h"
 #include "core/dom/custom/V0CustomElementObserver.h"
 #include "core/dom/custom/V0CustomElementScheduler.h"
+#include "core/frame/UseCounter.h"
 
 namespace blink {
 
@@ -108,6 +109,11 @@ void V0CustomElement::define(Element* element,
       break;
 
     case Element::V0WaitingForUpgrade:
+      UseCounter::count(
+          element->document(),
+          definition->descriptor().isTypeExtension()
+              ? UseCounter::V0CustomElementsCreateTypeExtensionElement
+              : UseCounter::V0CustomElementsCreateCustomTagElement);
       element->v0SetCustomElementDefinition(definition);
       V0CustomElementScheduler::scheduleCallback(
           definition->callbacks(), element,