When a site is installed and it pops into an app window, its history is still in the WebContents. This means pressing back may take a user back outside the scope which is a weird experience. We should do something reasonable here, possibly just erasing the tab's history stack (but this would be weird in a world where we envision a seamless app/tab pop-in/out experience).

Maybe we want back/forward to also affect the app vs tab state.