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When an HTTP auth prompt is cancelled, we close the prompt and refresh
the page to retrieve the 401 error body from the server. There is some
logic in LoginNavigationThrottle/LoginTabHelper that tracks/detects
these refreshes to handle them specially, i.e. not re-show an auth
prompt when the refresh commits. This logic was looking at
NavigationController:GetVisibleEntry() to determine when an observed
navigation was one of these refresh navigations -- but a visible entry
is not always guaranteed to exist, leading to crashes. One correct fix
would be to check for a null visible entry, but instead we now look at
the pending entry. This is more specific because the pending entry is
the one that would be for the refresh.

Note: this fix relies on the assumption that GetPendingEntry() will
always be non-null during a main-frame WillProcessResponse(). I think
this condition is guaranteed, but if I'm wrong, it will show up as
crashes when dereferencing GetPendingEntry(). In that case we'll need
to add a check for a null GetPendingEntry().

Bug: 1005096
Change-Id: If4a72ca903e14da588c0ee515648e8d2694f8612
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819331
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699074}