HttpAuthPromptType histogram records an entry in LoginDialogCallback each time it attempts to show a login prompt. However, not all auth requests end up displaying the dialog.

A login dialog is created for each request, but only one dialog is displayed per AuthInfo. E.g. if there are two subresources from the same origin and realm (but with different URLs), only one login dialog is actually displayed even though two are created. The other dialog is automatically closed based on the result of the first one.

So as it stands, Net.HttpAuthPromptType overcounts the number of HTTP auth dialogs being displayed.