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Currently, System-proxy requests proxy authentication credentials from
the Network Service. This means the user has to first to login into the
Browser and then use the Play Store for navigation.

This CL creates a proxy authentication dialogue for System-proxy to
request credentials for a remote proxy without going trough the Browser.

Bug:1112722
TEST=unittests

Change-Id: I263a38c8be4cbbdf0891e515500cd9e5584c63fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352828
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Cr-Commit-Position: refs/heads/master@{#799586}