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This is a code cleanup before changing ReloadWithUserAgentType() to work
also with WKBasedNavigationManager, which would not be appropriate to
add to tab.mm.

This CL also fixed an apparent bug where IsItemRedirectItem() in
tab.mm seemed to have always used the wrong polarity in detecting
redirect items.

Bug: 798836
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Id39b3788a566fc8b188bdceb3a02803e5e182364
Reviewed-on: https://chromium-review.googlesource.com/920101
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537113}