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Also fix a use case where user credentials are always being sent for
user traffic, regardless of the `ArcEnabled` setting.

Bug: 1139905
Test: unit test
Change-Id: Ia010653ea13cedd87049ca5537efbd107905151a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485091Reviewed-by: Andreea-Elena Costinas <acostinas@google.com>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Omar Morsi <omorsi@google.com>
Commit-Queue: Andreea-Elena Costinas <acostinas@google.com>
Cr-Commit-Position: refs/heads/master@{#820798}