Describe the bug
Embeddable URLs after single line breaks stop working

To Reproduce
Create a post from classic editor/pre-gutenberg with content:

Some text above
https://www.youtube.com/watch?v=3V9QHBgrPNY

https://www.youtube.com/watch?v=f5KyMNDJE6o

View it

Expected behavior
Two embedded videos

Screenshots
(can get if needed)

Actual behavior
One plain URL and one video

Cause

https://github.com/WordPress/gutenberg/blob/master/lib/compat.php#L117 runs before https://github.com/WordPress/WordPress/blob/master/wp-includes/class-wp-embed.php#L32

markup added by wpautop prevents regex from matching

Desktop (please complete the following information):

• OS: MacOS Mojave
• Browser Firefox
• Version 63.0.1

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context

• Gutenberg version: master branch
• WP Version: 4.9.8