The proxy auth details are transferred into the main request context on the chrome os sign-in screen

Original CL: https://codereview.chromium.org/9186039
Adapt for Webview: https://codereview.chromium.org/2644713003
Adapt for isolated StoragePartitions in Webview: 

We always wait 2s before doing the HttpAuthCache transfer.

Find a way to do it without a hard-coded delay.