Improve Spinner dismissing for challenge handling

[jasoncoolmax]

Currently for embedded webview, we dismiss the loading spinner in the following delegate callback:

• webView:didFinishNavigation:
• webView:didFailNavigation:withError:
• webView:didFailProvisionalNavigation:withError:

However, depending on server behavior, a page may or may not continue to load if we reject a auth challenge, say client TLS challenge.
If a page continues to load, the spinner will be dismissed by hitting any of the above delegate callbacks;
If a page does not continue to load, the spinner will not be dismissed.

We could try to figure out whether there is a way to improve the experience.