CSSUsage not finding all stylesheet content

Split out from https://github.com/GoogleChrome/lighthouse/issues/6628#issuecomment-485275120

#### Provide the steps to reproduce
1. Run LH on https://5cbc80f7b4528a0008a0dbad--lebikini.netlify.com/

#### What is the current behavior?

LH reports that the fonts don't have font-display properties. The CSSUsage artifact does not contain the content of any of the stylesheets.

#### What is the expected behavior?

LH does not report the fonts are missing `font-display`, and the CSSUsage artifact contains the content of the stylesheets.

#### Environment Information
* Affected Channels: CLI
* Lighthouse version: v4.3.0, v5.0.0

Reproduce the CLI report for https://5cbc80f7b4528a0008a0dbad--lebikini.netlify.com/ and inspect the CSSUsage artifact, which is reported to omit all stylesheet content. Confirm the current font-display audit output and trace how the artifact is populated. Done means the stylesheet content is present and Lighthouse no longer reports missing font-display properties for this page.

Written by the indexing model from the issue text.