Add a flag to identify Tor Browser in JS

When Tor Browser visits a WordPress site, they'll see canvas image data warning, as WordPress tries to detect if the current browser is capable of rendering emoji.

We can disable this test in Tor Browser, but we need a consistent method for testing that we're definitely in the Tor Browser. I don't believe such a flag would present a privacy issue, as it's already possible to detect if the user is coming from a Tor exit node with the Tor Browser's UA, but this method doesn't lend itself to WordPress' needs.

See further discussion on the WordPress ticket: https://core.trac.wordpress.org/ticket/32138

Trac:
Username: pento