Bug Description

Originally reported in #3928 (comment).

There is a conflict in WordPress 4.9 due to the amp-paired-browsing-client not being printed: https://travis-ci.org/ampproject/amp-wp/jobs/625330248#L1062-L1068

I believe this is due to the fact that amp-paired-browsing-client depends on both wp-dom-ready and wp-polyfill but only the former is being polyfilled for WordPress 4.9:

In other words, there is no polyfill for the polyfills.

Three options I see here:

1. Increase minimum WP version to 5.0 and eliminate these pre-Gutenberg polyfills altogether.
2. Polyfill the polyfills.
3. Prevent paired browsing from being available in WordPress 4.9.

I've gone ahead and removed the test case in 64a8051, but this will need to be fixed for v1.5 in a separate PR. That PR can revert this commit to re-add those tests.

Expected Behaviour

Paired browsing should either be not available or the minimum version of WordPress should be bumped.

Steps to reproduce

Try to access paired browsing in WordPress 4.9.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry