fix(ngMockE2E): ensure that mocked $httpBackend uses correct $browser

The fix from #13124 enabled ngMock and ngMockE2E to work together but
did it in a way that meant that the "real" `$httpBackend` service that
was used in pass-through depended upon a different `$browser` service
to the rest of the app.

This broke Protractor since it watches the `$browser` for outstanding
requests and the pass through requests were being tracked by the wrong
`$browser` instance.

Closes #15593