MenuItem: Use 'Close Window' for 'close' role label

On OS X, the standard label that's used for the 'close' role is 'Close
Window'. You can see this in the default macOS apps from Apple.

lib/browser/api/menu-item-roles.js

@@ -7,7 +7,7 @@ const roles = {
     }
   },
   close: {
-    label: 'Close',
+    label: process.platform === 'darwin' ? 'Close Window' : 'Close',
     accelerator: 'CommandOrControl+W',
     windowMethod: 'close'
   },