Don't change the URL of the main execution context on exit.

We shouldn't change the URL of the main execution context.
Because this will change security policy of our main frame which leads to the following message: `Unsafe JavaScript attempt to access frame with URL about:blank from frame with URL file://bla.js. Domains, protocols and ports must match.`

We can fix it by isolating main frame of PhantomJS execution context. We can destroy it separately.