We have considered official multiplayer features for C2, but I don't think it is worth implementing until there's good UDP (or an "unreliable mode" for transmission, which is likely spec-speak for UDP). As noted without this the latency spikes can make the game unplayable. I'm pretty sure even if Google are being quiet about it that they're working on something in this area though. WebRTC might also provide a solution.
As for tabbing away, it's not a new problem, fullscreen games could always randomly be quit or alt-tabbed in to the background, it's just a lot easier to do in a browser window. Use the 'request fullscreen' action and encourage the user to switch in to fullscreen, and that should increase their engagement and make an accidental quit less likely. Also, even inactive tabs still keep websocket connections active IIRC, it's just the browser will reduce the framerate to 1 FPS. So as long as your game can handle running at 1 FPS and not time out or lose track of the game state, it should be fine.