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Previously, with HTTP auth committed interstitials, LoginTabHelper
cancels navigations that receive 401 responses and commits a blank
page. The auth prompt is shown on top of the blank page
normally. However, in the case that the browser supplied credentials
and the server still sent a 401 response (i.e., credentials were
incorrect), LoginTabHelper would cancel the navigation but not show
the auth prompt (due to lack of auth challenge -- the challenge is
only populated when credentials aren't supplied in the request). This
led to a weird situation where neither the auth prompt nor the 401
response body from the server would be displayed to the user. To
rectify this, LoginTabHelper should not cancel a request with a 401
response unless there is an auth challenge present for which to show a
prompt. This maintains the same UX as we had before committed
interstitials.

Bug: 1047742
Change-Id: I83eba22e52676946d7f3a6cffc14f699265f75d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036970Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738298}