Make process.exit() quit gracefully

Instead of abrupting the whole program immediately, we should close all
windows and release all native resources gracefully on exit. This avoids
possible crashes.

Fix #3350.

atom/browser/browser.h

@@ -42,6 +42,9 @@ class Browser : public WindowListObserver {
   // Try to close all windows and quit the application.
   void Quit();
 
+  // Exit the application immediately and set exit code.
+  void Exit(int code);
+
   // Cleanup everything and shutdown the application gracefully.
   void Shutdown();