Stylesheet fetch error with external stylesheets

[conwaydev]

Bug Description

Enqueueing external stylesheets no longer works and I get a "Stylesheet fetch error" in the AMP validation:

Expected Behaviour

If I revert #4404 the validation works as expected and my styles are available on the AMP version.

Steps to reproduce

1. Set up AMP plugin in transition mode
2. Enqueue external stylesheets inside of a functions.php:

add_action( 'wp_enqueue_scripts', function () {
	wp_enqueue_style( 'seedlings', 'https://cms.sproutsocial.com/wp-content/themes/sprout/css/seedlings-marketing.css' );
	wp_enqueue_style( 'seedlings-cdn', 'https://media.sproutsocial.com/assets/insights/css/seedlings-marketing-7ef5e60862.css' );
} );

3. See the styles on the non-AMP versions
4. Update a page and see there are AMP validation issues with an error code of STYLESHEET_FETCH_ERROR

Screenshots

With 182634c9363b897fe154c0f9db1b0f2b18fe88f5 reset:

Additional context

• WordPress version: 5.4
• Plugin version: Version 1.6.0-alpha (Noticed on 1.5.1 on production)
• Gutenberg plugin version (if applicable): n/a
• AMP plugin template mode: n/a
• PHP version: 7.3
• OS: Linux (in a vagrant env)
• Browser: [e.g. chrome, safari] n/a
• Device: [e.g. iPhone6] n/a

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

Thank you. I can reproduce this issue. Will begin work shortly.

[westonruter]

@conwaydev Here’s a 1.5.2-RC1 pre-release build with the fix for you to test: amp.zip (v1.5.2-RC1-20200403T193031Z-3102c5dea)

[westonruter]

We've just published the 1.5.2 release to WordPress.org: https://wordpress.org/plugins/amp/

You can update now via the WordPress admin.

Release notes: https://github.com/ampproject/amp-wp/releases/tag/1.5.2

[conwaydev]

@westonruter as always thank you so much, y’all are the best