This change forwards to the WebView HTTP authentication credentials that
are supplied to an XHR from JavaScript. This allows the WebView to store these
credentials for use with later requests.
This is a re-working of https://android-git.corp.google.com/g/46430
Bug: 2544330
Change-Id: I3f04433b00233d7b7bf4f7e3471d8d15d8817f93