Enable feature detection of declarative Shadow DOM

To enable feature detection, this CL enables this:

  HTMLTemplateElement.prototype.hasOwnProperty("shadowRoot")

This is as documented in the explainer, at [1].

[1] https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md#feature-detection-and-polyfilling

Bug: 1042130
Change-Id: I529af9ce952b4d2e162e099c7ca703af8f3c4791
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166466
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763495}

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

@@ -34,4 +34,14 @@
     HTMLConstructor
 ] interface HTMLTemplateElement : HTMLElement {
     readonly attribute DocumentFragment content;
+
+    // This 'shadowRoot' attribute shadows Element.prototype.shadowRoot
+    // without changing any behavior. The existence of this attribute
+    // indicates support of Declarative Shadow DOM, so that this works:
+    //   function supportsDeclarativeShadowDOM() {
+    //     return HTMLTemplateElement.prototype.hasOwnProperty("shadowRoot");
+    //   }
+    // See https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md#feature-detection-and-polyfilling
+    // for more detail.
+    [RuntimeEnabled=DeclarativeShadowDOM, Affects=Nothing, ImplementedAs=OpenShadowRoot] readonly attribute ShadowRoot? shadowRoot;
 };

third_party/blink/web_tests/external/wpt/shadow-dom/declarative/declarative-shadow-dom-basic.tentative.html

@@ -26,6 +26,10 @@ test(() => {
   assert_array_equals(s1.assignedNodes(), [c1]);
 }, 'Declarative Shadow DOM: Basic test');
 
+test(() => {
+  assert_true(HTMLTemplateElement.prototype.hasOwnProperty("shadowRoot"),'Unable to feature detect');
+}, 'Declarative Shadow DOM: Feature detection');
+
 test(() => {
   const div = document.createElement('div');
   div.innerHTML = `

third_party/blink/web_tests/webexposed/global-interface-listing-expected.txt

@@ -4181,6 +4181,7 @@ interface HTMLTableSectionElement : HTMLElement
 interface HTMLTemplateElement : HTMLElement
     attribute @@toStringTag
     getter content
+    getter shadowRoot
     method constructor
 interface HTMLTextAreaElement : HTMLElement
     attribute @@toStringTag