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.


This fixes: #12697 #12660