Bug 264218

To reproduce:
1. Visit https://installed-pwa-display-mode.glitch.me on iOS
2. Install the page to the home screen and open it

Expected:
The page reports that the current display mode is "standalone"

Actual:
The page reports that the current display mode is "fullscreen"


Currently the spec states the following for the 'standalone' mode:
The web application is displayed to look and feel like a standalone native application. This can include the application having a different window, its own icon in the application launcher, etc. In this mode, the user agent excludes standard browser UI elements such as an URL bar, but standard system UI elements, such as window decorations, a system status bar, and/or a system back button, remain available.

This description sounds much more suitable for Safari on iOS,
which is still showing showing the status bar of the OS.

As opposed to the 'fullscreen' display mode, for which the spec states:
The web application is displayed with browser UI elements hidden and takes up the entirety of the available display area.