Use https://mail.google.com/mail as URL for content script errors

[sosnovsky]

Currently, error reports from the content script (labeled ex:s:gmail) have different URLs, such as https://mail.google.com/mail/u/0/#spam, https://mail.google.com/mail/u/0/, https://mail.google.com/mail/u/0/#inbox etc.
This causes identical errors to be reported as different ones due to different url property value.
To fix this, let's use the same url property value https://mail.google.com/mail for all errors from the ex:s:gmail environment.