Add browser argument to BrowserExtensionConnectedToServer event

Created by: ebrodymoore

Now that we're sending browser extension data to Sourcegraph.com, could we add which browser it's coming from?

To me, the most logical path would be to add browser: Chrome (or similar) as an argument to the event BrowserExtensionConnectedToServer. Does that seem like the best route for this?

I'm only interested in this data for Sourcegraph.com, but it would give us the option to add to pings at a later date.

@lguychard tagging you on this because it'd be in the web team domain (pun intended)