fix($browser): remove base href domain when url begins with '//'

This change prevents an incorrect appBase url from being calculated when the
<base> href's domain begins with '//'.

Related to #5292, tangentially.

I don't know for sure if this is the correct behaviour, but it doesn't seem
totally unreasonable.

Before this change, if an app used `<base href="//my.domain.com/app">`, the entire href would be treated as a path, and $location would behave incorrectly, due to appending it to the initial domain.

So, that's a thing.