Fix OnPageLoading and OnPageLoaded functions are not called from WKWe…

[Abhirasmanu-Trimble]

…bViewUIDelegate

Xamarin.Auth Pull Request

Fixes 1.

Checklist

• [ ] I have included examples or tests
• [ ] I have updated the change log
• [ ] I am listed in the CONTRIBUTORS file
• [ ] I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• WebAuthenticator.OnPageLoading method is called from DidStartProvisionalNavigation
• WebAuthenticator.OnPageLoaded method is called from DidFinishNavigation

[dnfclas]

All CLA requirements met.

[friendly-tech]

@Abhirasmanu-Trimble Do you know why this was not merged? Im seeing this issue with incomplete functionality when using code flow + WKWebView