If the back button goes back to the small chat I suppose that works for some users, but not as intuitive as a toggle button in my opinion. Doesn’t every chat (and other software) use a toggle button for that, as @geoff777 said?
By the way, I’m going to activate the chat feature in a few weeks for an imported and active forum that used to have a chat a long time ago and this feature is really missed by them. These users aren’t computer specialists (like, not at all 
) and it will be interesting to have their feedback when they’ll use the Discourse chat.