Tiny cleanup: Use `isContentEditable` property instead of attribute

Release-Notes: skip
Change-Id: I883d268e2a9003fdf9e788dcf98d10b20bb0fdb1
diff --git a/polygerrit-ui/app/utils/dom-util.ts b/polygerrit-ui/app/utils/dom-util.ts
index d557273..04e8c19 100644
--- a/polygerrit-ui/app/utils/dom-util.ts
+++ b/polygerrit-ui/app/utils/dom-util.ts
@@ -437,7 +437,7 @@
   if (!isElementTarget(rootTarget)) return false;
   const tagName = rootTarget.tagName;
   const type = rootTarget.getAttribute('type');
-  const editable = rootTarget.hasAttribute('contenteditable');
+  const editable = !!(rootTarget as HTMLElement).isContentEditable;
 
   if (
     editable ||