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This adds a switch so that an AuthServerWhitelist can be set in headless
mode which enables Negotiate authentication for the whitelisted URLs.
The switch is added to the HttpAuthPreferences for the HttpAuthHandlerFactory,
which is set in the URLRequestContext.

Bug: 741872
Change-Id: Iaeaefbf58ff21351b9830309faf4f017c14e7b17
Reviewed-on: https://chromium-review.googlesource.com/828321Reviewed-by: David Vallet <dvallet@chromium.org>
Commit-Queue: Iris Uy <irisu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524300}