Fix (again) the declarative Shadow DOM perf test

The [1] CL again changed the opt-in interface for declarative
Shadow DOM, and I forgot to update the perf test. This CL makes
that update. I wish there were perf bots in the CQ.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2530222

Bug: 1042130
Fixed: 1148583
Change-Id: Icee204fd4fecc2926dbe50ab7c08b36643d11c4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536451Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827188}

third_party/blink/perf_tests/parser/declarative-shadow-dom-cloning.html

@@ -4,8 +4,6 @@
 <script src="../resources/runner.js"></script>
 <script>
 
-const hostChildren = 100;
-
 function createDeepShadowTemplateContent(levels, slotted) {
   const nestedContent = levels > 0 ? createDeepShadowTemplateContent(levels-1) : '<span>End</span>';
   if (slotted) {
@@ -15,16 +13,19 @@ function createDeepShadowTemplateContent(levels, slotted) {
   }
 }
 
+const hostChildren = 100;
+
+function buildTemplate(slotted) {
+  const template = document.createElement('template');
+  document.body.appendChild(template);
+  const html = createDeepShadowTemplateContent(hostChildren, slotted);
+  template.setInnerHTML(html, {allowShadowRoot: true});
+  return template;
+}
+
 // Build templates once
-const templateSlotted = document.createElement('template');
-document.body.appendChild(templateSlotted);
-templateSlotted.content.allowDeclarativeShadowDom = true;
-templateSlotted.innerHTML = createDeepShadowTemplateContent(hostChildren, true);
-
-const templateNested = document.createElement('template');
-document.body.appendChild(templateNested);
-templateNested.content.allowDeclarativeShadowDom = true;
-templateNested.innerHTML = createDeepShadowTemplateContent(hostChildren, false);
+const templateSlotted = buildTemplate(true);
+const templateNested = buildTemplate(false);
 
 if (!templateSlotted.content.firstChild.shadowRoot) {
   PerfTestRunner.logFatalError("Declarative Shadow DOM must be enabled for this test.");