Simple browser example: create off-the-record profile in a lazy manner

Do not create multiple profiles immediately on start. Allows to start
and run browser with default profile with 'single-process' flag.

Change-Id: Idea7da8167152f718a3c2d4086ad4216d8e1b722
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>