UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36

Steps to reproduce the problem:
1. Launch chrome in default mode (non headless)
2. Send Emulation.setDeviceMetricsOverride via CDP
3. Navigate to https://www.youtube.com/watch?v=ZSQJbA9ZuL0 via CDP
4. Capture a screenshot via CDP
5. Observe incorrect viewport size

Here is a repro using puppeteer:
https://gist.github.com/kkoenig/84a87f1059d27167ee3eb86b0660f4f0

What is the expected behavior?
The screenshot viewport should remain the same regardless of the site contents

What went wrong?
The captured viewport size does not match the emulated viewport size for certain pages.

Did this work before? No 

Chrome version: 70.0.3538.102  Channel: stable
OS Version: Ubuntu 18.04.1 LTS
Flash Version: 

See the following CL by Jianzhou Feng:
https://chromium-review.googlesource.com/c/chromium/src/+/846611

Additional logic based on is_main_frame may be needed to handle the iframe case
 

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