Fix declarative Shadow DOM perf test

A previous CL added an opt-in for declarative Shadow DOM, and
this test needs to activate that opt in.

Fixed: 1146530
Change-Id: Iefa585f0a4c83208c6aa977fc65a85ebf6fb4a8d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523734
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825239}

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

@@ -18,10 +18,12 @@ function createDeepShadowTemplateContent(levels, slotted) {
 // 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);
+templateSlotted.content.allowDeclarativeShadowDom = true;
 templateNested.innerHTML = createDeepShadowTemplateContent(hostChildren, false);
 
 if (!templateSlotted.content.firstChild.shadowRoot) {