cooltaby As Ashley mentioned, use the browser objects to exit the game, its still working fine on Crosswalk.
Simple way is this: Browser - On Back button pressed - Browser - Close
You could also create a sprite and when pressed, trigger the Browser 'Close' action.
What i did was if the back button was pressed, a pop up will appear asking for confirmation to exit game. If yes is pressed then the Browser 'Close' will trigger, exiting the game. If no is pressed, then the pop-ups disappear and you get back to the game.