Provides ability to store HTTP authentication credentials supplied to an XHR from JavaScript

This change forwards to the WebViewClient HTTP authentication credentials that
are supplied to an XHR from JavaScript. This allows the client to store these
credentials for use with later requests.

Currently, the browser only stores credentials that have been entered manually
by the user through the authentication dialog.

Bug: 2544330
Change-Id: I913e35d80b7ad41ff75586092408b86ea9f543d8