Move declarative Shadow DOM use counter into element.cc

It makes more sense to count this feature at the usage site,
which is Element::AttachDeclarativeShadowRoot.

Bug: 1042130
Change-Id: I53ffbf6821b56e964dc5ab1eae84a0ed566a96e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227338
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774441}

third_party/blink/renderer/core/dom/element.cc

@@ -3674,6 +3674,7 @@ void Element::AttachDeclarativeShadowRoot(HTMLTemplateElement* template_element,
                                           SlotAssignmentMode slot_assignment) {
   DCHECK(template_element);
   DCHECK(type == ShadowRootType::kOpen || type == ShadowRootType::kClosed);
+  UseCounter::Count(GetDocument(), WebFeature::kDeclarativeShadowRoot);
 
   // 12. Run attach a shadow root with shadow host equal to declarative shadow
   // host element, mode equal to declarative shadow mode, and delegates focus

third_party/blink/renderer/core/html/parser/html_tree_builder.cc

@@ -957,8 +957,6 @@ bool HTMLTreeBuilder::ProcessTemplateEndTag(AtomicHTMLToken* token) {
         shadow_host_stack_item->GetNode() != tree_.OpenElements()->RootNode()) {
       DCHECK(shadow_host_stack_item);
       DCHECK(shadow_host_stack_item->IsElementNode());
-      UseCounter::Count(shadow_host_stack_item->GetElement()->GetDocument(),
-                        WebFeature::kDeclarativeShadowRoot);
       bool delegates_focus = template_stack_item->GetAttributeItem(
           html_names::kShadowrootdelegatesfocusAttr);
       // TODO(crbug.com/1063157): Add an attribute for imperative slot