Add ChromeOS support in defaultPlatform frontmatter

[sirocyl]

Adds CrOS to defaultPlatform frontmatter, as a variant of Linux.

Why:

As noted in #4101, ChromeOS is not identified in pages which have multiple platform-specific parts.

What's being changed:

This change:

• adds "cros" to the list of user agent matches done in user-agent.js, and
• maps "cros" to linux, in order to adjust the effective platform name, in display-platform-specific-content.js.

Check off the following:

• I have reviewed my changes in staging. (Works as expected in ChromeOS.)
• [n/a] For content changes, I have reviewed the localization checklist
• [n/a] For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

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[janiceilene]

@sirocyl Thanks so much for opening a PR! I'll get this triaged for review ⚡

[heiskr]

Neat 💃🏼

[heiskr]

Oh, I think we'll need to update our event schema too; so that analytics events coming from ChromeOS pass the validation.

https://github.com/sirocyl/docs/blob/main/lib/schema-event.js#L76

[github-actions]

A stale label has been added to this pull request because it has been open 7 days with no activity. To keep this PR open, add a comment or push a commit within 3 days.

[janiceilene]

I don't think this one's stale! Reopening.

[sirocyl]

Just checking in - is there anything I need to do here, or are we just waiting to move through the pipeline?

[github-actions]

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