Explicitly set "available to element internals" for declarative

Per some recent spec discussions/changes, declarative shadow
roots should manually set the "available to element internals"
flag, rather than assuming that in the check. See [1].

[1] https://github.com/whatwg/html/pull/5912#discussion_r489750553

Bug: 1042130
Change-Id: If50fa8bff4cddb566edf95cbd1b1a8f1afe11b49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429852
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@{#810930}

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

@@ -3560,6 +3560,10 @@ void Element::AttachDeclarativeShadowRoot(HTMLTemplateElement* template_element,
   // 13.1. Set declarative shadow host element's shadow host's "is declarative
   // shadow root" property to true.
   shadow_root.SetIsDeclarativeShadowRoot(true);
+  // 13.NEW. Set declarative shadow host element's shadow host's "available
+  // to element internals" to true.
+  shadow_root.SetAvailableToElementInternals(true);
+
   // 13.2. Append the declarative template element's DocumentFragment to the
   // newly-created shadow root.
   shadow_root.appendChild(template_element->DeclarativeShadowContent());

third_party/blink/renderer/core/dom/shadow_root.h

@@ -169,7 +169,7 @@ class CORE_EXPORT ShadowRoot final : public DocumentFragment, public TreeScope {
     available_to_element_internals_ = flag;
   }
   bool IsAvailableToElementInternals() const {
-    return available_to_element_internals_ || is_declarative_shadow_root_;
+    return available_to_element_internals_;
   }
 
   bool ContainsShadowRoots() const { return child_shadow_root_count_; }