Use window.ontouchstart to detect Safari on iPad.

[denschub]

One-line summary

With yesterday's release of Safari Tech Preview 173, Safari on macOS now supports Web Apps. This results in navigator.standalone existing, which in turn means that Safari on macOS users will be prompted to install the iOS version of Firefox on https://www.mozilla.org/en-US/firefox/new/

Significant changes and points to review

This PR changes that to look for window.ontouchstart instead, as Safari on macOS does not support those. Thanks to @nt1m for that suggestion.

Issue / Bugzilla link

n/a

Screenshots

n/a

Checklist

If relevant:

• Tests added
• Feature flags added to www-config
• New secrets added to the secrets repository
• If new dependencies are added, I've checked their license is appropriate

Testing

Demo server URL: (or None)

To test this work:

• Open /en-US/firefox/new/ in Safari Tech Preview 173 on macOS and verify that the user is promoted to download the Mac versoin.

[zcorpan]

FYI https://w3c.github.io/touch-events/#dfn-expose-legacy-touch-event-apis is defined but only used for hiding ontouchstart et al event handlers, not window.TouchEvent (per spec). Also see whatwg/dom#952

[denschub]

I changed the patch to check for ontouchstart instead. :)

[alexgibson]

Confirmed this works locally with Safari tech preview and iOS simulator.

Thanks for the report, and the fix! 💯