[WebAPI] Implement navigator.pdfViewerEnabled

[Rumyra]

Acceptance Criteria

• The listed features are documented sufficiently on MDN
• BCD is updated
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes if feature is enabled in release
  or
• Add entry to Firefox experimental features page if feature is not yet enabled in release

Features to document

Implement navigator.pdfViewerEnabled / hard-coded navigator.plugins + navigator.mimeTypes

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1720353

Other

[hamishwillee]

For FF this updates spec to implement behaviour described in https://html.spec.whatwg.org/multipage/system-state.html#pdf-viewing-support.

In essence people were using a deprecated API to find work out whether inline PDF display was working based on the supported mime types and a small number of plugins they trusted. This was unreliable, and provided a fingerprinting surface. So this change provides a new API to just query whether it is supported or not, and also updates the old API so it returns the same information for all browser (i.e. no fingerprinting) and this matches what sites are normally using to determine inline browsing support.

The new property is navigator.pdfViewerEnabled. It is in BCD because was added in Chrome 94. Info about this and other platforms here. It is not documented.

The deprecated properties are Navigator.mimeTypes and Navigator.plugins . These exist but need work.

I am confirming the above in https://bugzilla.mozilla.org/show_bug.cgi?id=1720353#c18

[hamishwillee]

Status

• BCD: FF98 Navigator.pdfViewerEnabled - supported browser-compat-data#15292
• Docs: Navigator.plugins, mimeTypes, pdfViewerEnabled updates #13605
• Release note - FF99 ReleaseNote: Navigator.pdfViewerEnabled #13628
• dev-docs-complete