Just ensure content frag from _contentForTemplate is inert.

Edge does not seem to always use the exact same owner document for templates.

test/unit/template-stamp.html

@@ -74,7 +74,7 @@
         document.body.appendChild(el);
         let content = el.constructor._contentForTemplate(el.dom.$.nestedTemplate);
         assert.isTrue(content instanceof DocumentFragment);
-        assert.equal(content.ownerDocument, el.dom.$.nestedTemplate.content.ownerDocument);
+        assert.notOk(content.ownerDocument.defaultView);
         assert.equal(content.firstChild.getAttribute('on-click'), null);
         document.body.removeChild(el);
       });