Add UMA for HTMLTemplateElement usage

Add UMA for any HTMLTemplateElement usage.
We might want to add other UMA for different usages,
as https://crbug.com/922719#c1 requested, later.

Bug: 922719
Change-Id: Ib5f3c1a1bc573fb290d52a73ecbca38a5b86b8fa
Reviewed-on: https://chromium-review.googlesource.com/c/1436728Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626452}

third_party/blink/public/platform/web_feature.mojom

@@ -2201,6 +2201,7 @@ enum WebFeature {
   kV8RTCQuicStream_Reset_Method = 2766,
   kV8RTCQuicStream_WaitForWriteBufferedAmountBelow_Method = 2767,
   kV8RTCQuicStream_WaitForReadable_Method = 2768,
+  kHTMLTemplateElement = 2769,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/html/html_template_element.cc

@@ -33,13 +33,16 @@
 #include "third_party/blink/renderer/core/dom/document.h"
 #include "third_party/blink/renderer/core/dom/document_fragment.h"
 #include "third_party/blink/renderer/core/dom/template_content_document_fragment.h"
+#include "third_party/blink/renderer/core/frame/use_counter.h"
 
 namespace blink {
 
 using namespace html_names;
 
 inline HTMLTemplateElement::HTMLTemplateElement(Document& document)
-    : HTMLElement(kTemplateTag, document) {}
+    : HTMLElement(kTemplateTag, document) {
+  UseCounter::Count(document, WebFeature::kHTMLTemplateElement);
+}
 
 DEFINE_NODE_FACTORY(HTMLTemplateElement)
 

tools/metrics/histograms/enums.xml

@@ -21422,6 +21422,7 @@ Called by update_net_error_codes.py.-->
   <int value="2767"
       label="V8RTCQuicStream_WaitForWriteBufferedAmountBelow_Method"/>
   <int value="2768" label="V8RTCQuicStream_WaitForReadable_Method"/>
+  <int value="2769" label="HTMLTemplateElement"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">