Pass the browser to TorDomainIsolator.newCircuitForBrowser

Currently, we pass gBrowser to all the calls of TorDomainIsolator.newCircuitForBrowser.

I don't remember if the reason is that this was always this way, or if I did to avoid having to repeat gBrowser.selectedBrowser at every call.

However, we now need to pass the actual browser for #42655 (closed) (the workaround would be to pass { selectedBrowser: this.browser } there, which would be extremely ugly).