Issue 494350655: Local Core Web Vitals flicker with iframe activity

Expected

The Local metric card should reflect only the primary frame and remain stable while nested iframes update.

Actual (regressed builds)

When iframe contexts emit reset/LCP events, Local metric values can flicker, disappear, or show wrong values.

1. Open this page in Chrome with DevTools open.

2. Go to Performance panel and watch the Local metrics card.

3. Click Start iframe churn below.

4. In regressed builds, LCP/Local values flicker or blank out every iframe reload.

5. In fixed builds, metrics stay tied to the top frame and do not reset from iframe activity.