"Untrusted viewer url override" error emitted for every web view page load

This affects the iOS Google app.

We hit this line every time
https://github.com/ampproject/amphtml/blob/f28e1169aaf7732d26a17cab469cd377a0374265/src/service/viewer-impl.js#L373

Is that a problem? Should the severity go down?

I also noticed that there are no callers of the `getViewerUrl` method.

CC @erwinmombay @dvoytenko 
