Fullscreen state lost on page reload

When the page reloads while in fullscreen mode, the window.__ELECTRON_IS_FULLSCREEN__ variable is lost since it's stored on the window object which is recreated. The did-finish-load handler creates the drag bar without checking current fullscreen state, and useElectronFullscreen initializes to false since the variable is undefined. The enter-full-screen event won't re-fire because the window is already fullscreen. This results in the UI showing non-fullscreen state (visible spacer and drag bar) until the user exits and re-enters fullscreen. The main process could inject the current state by calling win.isFullScreen() in the did-finish-load handler.