Main thread CPU reporting for iframes

#### Provide the steps to reproduce
1. Run LH on https://www.concatencate.dev

#### What is the current behavior?
YouTube's iframe is reported to be blocking the main thread for ~1 second

#### What is the expected behavior?
YouTube is most probably blocking the main thread, but the main thread of the iframe. I'd expect it to be reported somewhat differently.

#### Environment Information
https://developers.google.com/speed/pagespeed/insights/?url=https%3A%2F%2Fconcatenate.dev&tab=mobile

**Related issues**
https://github.com/GoogleChrome/lighthouse/issues/213 mentioned that, but I'm not sure if it eventually resolved it or not.