Enable Brotli compression for .onion domains

Tor Browser treats .onion as secure domains. Brotli compression is only enabled in Firefox on secure domains, but not for .onion domains.

Internally, Firefox controls these from the following settings: network.http.accept-encoding network.http.accept-encoding.secure

.onion is treated as the first instance (insecure) and only enable gzip and deflate. It should be handled as the second category and thus also enable Brotli compression.

Brotli compression will be beneficial to .onion service performance and reducing the data usage of Tor Browser.

PS: The requirement for Brotli to only be used on secure connections was a political decision by Google who wanted to use their new efficient compression method as a carrot to encourage HTTPS adoption.

Trac:
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